System and method for generating signal waveforms in a CDMA cellular telephone system

ABSTRACT

A system and method for communicating information signals using spread spectrum communication techniques. PN sequences are constructed that provide orthogonality between the users so that mutual interference will be reduced, allowing higher capacity and better link performance. With orthogonal PN codes, the cross-correlation is zero over a predetermined time interval, resulting in no interference between the orthogonal codes, provided only that the code time frames are time aligned with each other. In an exemplary embodiment, signals are communicated between a cell-site and mobile units using direct sequence spread spectrum communication signals. In the cell-to-mobile link, pilot, sync, paging and voice channels are defined. Information communicated on the cell-to-mobile link channels are, in general, encoded, interleaved, bi-phase shift key (BPSK) modulated with orthogonal covering of each BPSK symbol along with quadrature phase shift key (QPSK) spreading of the covered symbols. In the mobile-to-cell link, access and voice channels are defined. Information communicated on the mobile-to-cell link channels are, in general, encoded, interleaved, orthogonal signaling along with QPSK spreading.

[0001] This is a continuation of U.S. patent application Ser. No.09/360,059, filed Jul. 23, 1999, now U.S. Pat. No. ______ issued ______,which is a continuation of U.S. patent application Ser. No. 08/441,895,filed May 16, 1995, now U.S. Pat. No. 5,943,361 issued Aug. 24, 1999,which is a continuation of U.S. patent application Ser. No. 07/825,147,filed Jan. 24, 1992, now U.S. Pat. No. 5,416,797 issued May 16, 1995,which is a continuation of U.S. patent application Ser. No. 07/543,496,filed Jun. 25, 1990, now U.S. Pat. No. 5,103,459 issued Apr. 7, 1992,all of which are entitled “SYSTEM AND METHOD FOR GENERATING SIGNALWAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEM”. The present inventionrelates to cellular telephone systems. More specifically, the presentinvention relates to a novel and improved system and method forcommunicating information, in a mobile cellular telephone system orsatellite mobile telephone system, using spread spectrum communicationsignals.

BACKGROUND OF THE INVENTION

[0002] I. Field of the Invention

[0003] II. Description of the Related Art

[0004] The use of code division multiple access (CDMA) modulationtechniques is one of several techniques for facilitating communicationsin which a large number of system users are present. Other multipleaccess communication system techniques, such as time division multipleaccess (TDMA), frequency division multiple access (FDMA) and AMmodulation schemes such as amplitude companded single sideband (ACSSB)are known in the art. However the spread spectrum modulation techniqueof CDMA has significant advantages over these modulation techniques formultiple access communication systems. The use of CDMA techniques in amultiple access communication system is disclosed in U.S. Pat. No.4,901,307, entitled “SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATIONSYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS”, assigned to theassignee of the present invention, of which the disclosure thereof isincorporated by reference.

[0005] In the just mentioned patent, a multiple access technique isdisclosed where a large number of mobile telephone system users eachhaving a transceiver communicate through satellite repeaters orterrestrial base stations (also referred to as cell-sites stations,cell-sites or for short, cells) using code division multiple access(CDMA) spread spectrum communication signals. In using CDMAcommunications, the frequency spectrum can be reused multiple times thuspermitting an increase in system user capacity. The use of CDMA resultsin a much higher spectral efficiency than can be achieved using othermultiple access techniques.

[0006] The satellite channel typically experiences fading that ischaracterized as Rician. Accordingly the received signal consists of adirect component summed with a multiple reflected component havingRayleigh fading statistics. The power ratio between the direct andreflected component is typically on the order of 6-10 dB, depending uponthe characteristics of the mobile unit antenna and the environment aboutthe mobile unit.

[0007] Contrasting with the satellite channel, the terrestrial channelexperiences signal fading that typically consists of the Rayleigh fadedcomponent without a direct component. Thus, the terrestrial channelpresents a more severe fading environment than the satellite channel inwhich Rician fading is the dominant fading characteristic.

[0008] The Rayleigh fading characteristic in the terrestrial channelsignal is caused by the signal being reflected from many differentfeatures of the physical environment. As a result, a signal arrives at amobile unit receiver from many directions with different transmissiondelays. At the UHF frequency bands usually employed for mobile radiocommunications, including those of cellular mobile telephone systems,significant phase differences in signals traveling on different pathsmay occur. The possibility for destructive summation of the signals mayresult, with on occasion deep fades occurring.

[0009] Terrestrial channel fading is a very strong function of thephysical position of the mobile unit. A small change in position of themobile unit changes the physical delays of all the signal propagationpaths, which further results in a different phase for each path. Thus,the motion of the mobile unit through the environment can result in aquite rapid fading process. For example, in the 850 MHz cellular radiofrequency band, this fading can typically be as fast as one fade persecond per mile per hour of vehicle speed. Fading this severe can beextremely disruptive to signals in the terrestrial channel resulting inpoor communication quality. Additional transmitter power can be used toovercome the problem of fading. However, such power increases effectboth the user, in excessive power consumption, and the system byincreased interference.

[0010] The CDMA modulation techniques disclosed in U.S. Pat. No.4,901,307 offer many advantages over narrow band modulation techniquesused in communication systems employing satellite or terrestrialrepeaters. The terrestrial channel poses special problems to anycommunication system particularly with respect to multipath signals. Theuse of CDMA techniques permit the special problems of the terrestrialchannel to be overcome by mitigating the adverse effect of multipath,e.g. fading, while also exploiting the advantages thereof.

[0011] In a CDMA cellular telephone system, the same frequency band canbe used for communication in all cells. The CDMA waveform propertiesthat provide processing gain are also used to discriminate betweensignals that occupy the same frequency band. Furthermore the high speedpseudonoise (PN) modulation allows many different propagation paths tobe separated, provided the difference in path delays exceed the PN chipduration, i.e. 1/bandwidth. If a PN chip rate of approximately 1 MHz isemployed in a CDMA system, the full spread spectrum processing gain,equal to the ratio of the spread bandwidth to system data rate, can beemployed against paths that differ by more than one microsecond in pathdelay from the desired path. A one microsecond path delay differentialcorresponds to differential path distance of approximately 1,000 feet.The urban environment typically provides differential path delays inexcess of one microsecond, and up to 10-20 microseconds are reported insome areas.

[0012] In narrow band modulation systems such as the analog FMmodulation employed by conventional telephone systems, the existence ofmultiple paths results in severe multipath fading. With wide band CDMAmodulation, however, the different paths may be discriminated against inthe demodulation process. This discrimination greatly reduces theseverity of multipath fading. Multipath fading is not totally eliminatedin using CDMA discrimination techniques because there will occasionallyexist paths with delayed differentials of less than the PN chip durationfor the particular system. Signals having path delays on this ordercannot be discriminated against in the demodulator, resulting in somedegree of fading.

[0013] It is therefore desirable that some form of diversity be providedwhich would permit a system to reduce fading. Diversity is one approachfor mitigating the deleterious effects of fading. Three major types ofdiversity exist: time diversity, frequency diversity and spacediversity.

[0014] Time diversity can best be obtained by the use of repetition,time interleaving, and error detection and coding which is a form ofrepetition. The present invention employs each of these techniques as aform of time diversity.

[0015] CDMA by its inherent nature of being a wideband signal offers aform of frequency diversity by spreading the signal energy over a widebandwidth. Therefore, frequency selective fading affects only a smallpart of the CDMA signal bandwidth.

[0016] Space or path diversity is obtained by providing multiple signalpaths through simultaneous links from a mobile user through two or morecell-sites. Furthermore, path diversity may be obtained by exploitingthe multipath environment through spread spectrum processing by allowinga signal arriving with different propagation delays to be received andprocessed separately. Examples of path diversity are illustrated in U.S.Pat. No. 5,101,501, entitled “SOFT HANDOFF IN A CDMA CELLULAR TELEPHONESYSTEM”, and U.S. Pat. No. 5,109,390, entitled “DIVERSITY RECEIVER IN ACDMA CELLULAR TELEPHONE SYSTEM”, both assigned to the assignee of thepresent invention.

[0017] The deleterious effects of fading can be further controlled to acertain extent in a CDMA system by controlling transmitter power. Asystem for cell-site and mobile unit power control is disclosed in U.S.Pat. No. 5,056,109, entitled “METHOD AND APPARATUS FOR CONTROLLINGTRANSMISSION POWER IN A CDMA CELLULAR MOBILE TELEPHONE SYSTEM”, alsoassigned to the assignee of the present invention.

[0018] The CDMA techniques as disclosed in U.S. Pat. No. 4,901,307contemplated the use of coherent modulation and demodulation for bothdirections of the link in mobile-satellite communications. Accordingly,disclosed therein is the use of a pilot carrier signal as a coherentphase reference for the satellite-to-mobile link and the cell-to-mobilelink. In the terrestrial cellular environment, however, the severity ofmultipath fading, with the resulting phase disruption of the channel,precludes usage of coherent demodulation technique for themobile-to-cell link. The present invention provides a means forovercoming the adverse effects of multipath in the mobile-to-cell linkby using noncoherent modulation and demodulation techniques.

[0019] The CDMA techniques as disclosed in U.S. Pat. No. 4,901,307further contemplated the use of relatively long PN sequences with eachuser channel being assigned a different PN sequence. Thecross-correlation between different PN sequences and the autocorrelationof a PN sequence for all time shifts other than zero both have a zeroaverage value which allows the different user signals to bediscriminated upon reception.

[0020] However, such PN signals are not orthogonal. Although thecross-correlations average to zero, for a short time interval such as aninformation bit time the cross-correlation follows a binomialdistribution. As such, the signals interfere with each other much thesame as if they were wide bandwidth Gaussian noise at the same powerspectral density. Thus the other user signals, or mutual interferencenoise, ultimately limits the achievable capacity.

[0021] The existence of multipath can provide path diversity to awideband PN CDMA system. If two or more paths are available with greaterthan one microsecond differential path delay, two or more PN receiverscan be employed to separately receive these signals. Since these signalswill typically exhibit independence in multipath fading, i.e., theyusually do not fade together, the outputs of the two receivers can bediversity combined. Therefore a loss in performance only occurs whenboth receivers experience fades at the same time. Hence, one aspect ofthe present invention is the provision of two or more PN receivers incombination with a diversity combiner. In order to exploit the existenceof multipath signals, to overcome fading, it is necessary to utilize awaveform that permits path diversity combining operations to beperformed.

[0022] It is therefore an object of the present invention to provide forthe generation of PN sequences which are orthogonal so as to reducemutual interference, thereby permitting greater user capacity, andsupport path diversity thereby overcoming fading.

SUMMARY OF THE INVENTION

[0023] The implementation of spread spectrum communication techniques,particularly CDMA techniques, in the mobile cellular telephoneenvironment therefore provides features which vastly enhance systemreliability and capacity over other communication system techniques.CDMA techniques as previously mentioned further enable problems such asfading and interference to be readily overcome. Accordingly, CDMAtechniques further promote greater frequency reuse, thus enabling asubstantial increase in the number of system users.

[0024] The present invention is a novel and improved method and systemfor constructing PN sequences that provide orthogonality between theusers so that mutual interference will be reduced, allowing highercapacity and better link performance. With orthogonal PN codes, thecross-correlation is zero over a predetermined time interval, resultingin no interference between the orthogonal codes, provided only that thecode time frames are time aligned with each other.

[0025] In an exemplary embodiment, signals are communicated between acell-site and mobile units using direct sequence spread spectrumcommunication signals. In the cell-to-mobile link, pilot, sync, pagingand voice channels are defined. Information communicated on thecell-to-mobile link channels are, in general, encoded, interleaved,bi-phase shift key (BPSK) modulated with orthogonal covering of eachBPSK symbol along with quadrature phase shift key (QPSK) spreading ofthe covered symbols.

[0026] In the mobile-to-cell link, access and voice channels aredefined. Information communicated on the mobile-to-cell link channelsare, in general, encoded, interleaved, orthogonal signaling along withQPSK spreading.

BRIEF DESCRIPTION OF THE DRAWINGS

[0027] The features, objects, and advantages of the present inventionwill become more apparent from the detailed description set forth belowwhen taken in conjunction with the drawings in which like referencecharacters identify correspondingly throughout and wherein:

[0028]FIG. 1 is a schematic overview of an exemplary CDMA cellulartelephone system;

[0029]FIG. 2 is a block diagram of the cell-site equipment asimplemented in the CDMA cellular telephone system;

[0030]FIG. 3 is a block diagram of the cell-site receiver;

[0031]FIGS. 4a-4 c is a block diagram of the cell-site transmitmodulator; and

[0032]FIG. 5 is an exemplary timing diagram of sync channel symbolsynchronization;

[0033]FIG. 6 is an exemplary timing diagram of sync channel timing withorthogonal covering;

[0034]FIG. 7 is an exemplary timing diagram of the overallcell-to-mobile link timing;

[0035]FIG. 8 is a block diagram of the mobile telephone switching officeequipment;

[0036]FIG. 9 is a block diagram of the mobile unit telephone configuredfor CDMA communications in the CDMA cellular telephone system;

[0037]FIG. 10 is a block diagram of the mobile unit receiver; and

[0038]FIG. 11 is a block diagram of the mobile unit transmit modulator;

[0039]FIG. 12 is an exemplary timing diagram of the mobile-to-cell linkfor the variable data rate with burst transmission; and

[0040]FIG. 13 is an exemplary timing diagram of the overallmobile-to-cell link timing.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0041] In a CDMA cellular telephone system, each cell-site has aplurality of modulator-demodulator units or spread spectrum modems. Eachmodem consists of a digital spread spectrum transmit modulator, at leastone digital spread spectrum data receiver and a searcher receiver. Eachmodem at the cell-site is assigned to a mobile unit as needed tofacilitate communications with the assigned mobile unit.

[0042] A soft handoff scheme is employed for a CDMA cellular telephonesystem in which a new cell-site modem is assigned to a mobile unit whilethe old cell-site modem continues to service the call. When the mobileunit is located in the transition region between the two cell-sites, thecall can be switched back and forth between cell-sites as signalstrength dictates. Since the mobile unit is always communicating throughat least one cell-site modem, fewer disrupting effects to the mobileunit or in service will occur. The mobile unit thus utilizes multiplereceivers for assisting in the handoff process in addition to adiversity function for mitigating the effects of fading.

[0043] In the CDMA cellular telephone system, each cell-site transmits a“pilot carrier” signal. Should the cell be divided into sectors, eachsector has an associated distinct pilot signal within the cell. Thispilot signal is used by the mobile units to obtain initial systemsynchronization and to provide robust time, frequency and phase trackingof the cell-site transmitted signals. Each cell-site also transmitsspread spectrum modulated information, such as cell-site identification,system timing, mobile paging information and various other controlsignals.

[0044] The pilot signal transmitted by each sector of each cell is ofthe same spreading code but with a different code phase offset. Phaseoffset allows the pilot signals to be distinguished from one anotherthus distinguishing originating cell-sites or sectors. Use of the samepilot signal code allows the mobile unit to find system timingsynchronization by a single search through all pilot signal code phases.The strongest pilot signal, as determined by a correlation process foreach code phase, is readily identifiable. The identified strongest pilotsignal generally corresponds to the pilot signal transmitted by thenearest cell-site. However, the strongest pilot signal is used whetheror not it is transmitted by the closest cell-site.

[0045] Upon acquisition of the strongest pilot signal, i.e. initialsynchronization of the mobile unit with the strongest pilot signal, themobile unit searches for another carrier intended to be received by allsystem users in the cell. This carrier, called the synchronizationchannel, transmits a broadcast message containing system information foruse by the mobiles in the system. The system information identifies thecell-site and the system in addition to conveying information whichallows the long PN codes, interleaver frames, vocoders and other systemtiming information used by the mobile unit to be synchronized withoutadditional searching. Another channel, called the paging channel mayalso be provided to transmit messages to mobiles indicating that a callhas arrived for them, and to respond with channel assignments when amobile initiates a call.

[0046] The mobile unit continues to scan the received pilot carriersignal code at the code offsets corresponding to cell-site neighboringsector or neighboring transmitted pilot signals. This scanning is donein order to determine if a pilot signal emanating from a neighboringsector or cell is becoming stronger than the pilot signal firstdetermined to be strongest. If, while in this call inactive mode, aneighbor sector or neighbor cell-site pilot signal becomes stronger thanthat of the initial cell-site sector or cell-site transmitted pilotsignal, the mobile unit will acquire the stronger pilot signals andcorresponding sync and paging channel of the new sector or cell-site.

[0047] When a call is initiated, a pseudonoise (PN) code address isdetermined for use during the course of this call. The code address maybe either assigned by the cell-site or be determined by prearrangementbased upon the identity of the mobile unit. After a call is initiatedthe mobile unit continues to scan the pilot signal transmitted by thecell-site through which communications are established in addition topilot signal of neighboring sectors or cells. Pilot signal scanningcontinues in order to determine if one of the neighboring sector or celltransmitted pilot signals becomes stronger than the pilot signaltransmitted by the cell-site the mobile unit is in communication with.When the pilot signal associated with a neighboring cell or cell sectorbecomes stronger than the pilot signal of the current cell or cellsector, it is an indication to the mobile unit that a new cell or cellsector has been entered and that a handoff should be initiated.

[0048] An exemplary telephone system in which the present invention isembodied is illustrated in FIG. 1. The system illustrated in FIG. 1utilizes spread spectrum modulation techniques in communication betweenthe system mobile units or mobile telephones, and the cell-sites.Cellular systems in large cities may have hundreds of cell-site stationsserving hundreds of thousands of mobile telephones. The use of spreadspectrum techniques, in particular CDMA, readily facilitates increasesin user capacity in systems of this size as compared to conventional FMmodulation cellular systems.

[0049] In FIG. 1, system controller and switch 10, also referred to asmobile telephone switching office (MTSO), typically includes interfaceand processing circuitry for providing system control to the cell-sites.Controller 10 also controls the routing of telephone calls from thepublic switched telephone network (PSTN) to the appropriate cell-sitefor transmission to the appropriate mobile unit. Controller 10 alsocontrols the routing of calls from the mobile units, via at least onecell-site, to the PSTN. Controller 10 may connect calls between mobileusers via the appropriate cell-sites since the mobile units do nottypically communicate directly with one another.

[0050] Controller 10 may be coupled to the cell-sites by various meanssuch as dedicated telephone lines, optical fiber links or microwavecommunication links. In FIG. 1, two such exemplary cell-sites 12 and 14including, along with mobile units 16 and 18 each including a cellulartelephone are illustrated. Cell-sites 12 and 14 as discussed herein andillustrated in the drawings are considered to service an entire cell.However it should be understood that the cell may be geographicallydivided into sectors with each sector treated as a different coveragearea. Accordingly, handoffs are made between sectors of a same cell asis described herein for multiple cells, while diversity may also beachieved between sectors as is for cells.

[0051] In FIG. 1, arrowed lines 20 a-20 b and 22 a-22 b respectivelydefine the possible communication links between cell-site 12 and mobileunit 16 and 18. Similarly, arrowed lines 24 a-24 b and 26 a-26 brespectively define the possible communication links between cell-site14 and mobile units 16 and 18. Cell-sites 12 and 14 nominally transmitusing equal power.

[0052] The cell-site service areas or cells are designed in geographicshapes such that the mobile unit will normally be closest to onecell-site, and within one cell sector should the cell be divided intosectors. When the mobile unit is idle, i.e. no calls in progress, themobile unit constantly monitors the pilot signal transmissions from eachnearby cell-site, and if applicable from a single cell-site in which thecell is sectorized. As illustrated in FIG. 1, the pilot signals arerespectively transmitted to mobile unit 16 by cell-sites 12 and 14 uponoutbound or forward communication links 20 a and 26 a. Mobile unit 16can determine which cell it is in by comparing signal strength in pilotsignals transmitted from cell-sites 12 and 14.

[0053] In the example illustrated in FIG. 1, mobile unit 16 may beconsidered closest to cell-site 12. When mobile unit 16 initiates acall, a control message is transmitted to the nearest cell-site,cell-site 12. Cell-site 12 upon receiving the call request message,transfers the called number to system controller 10. System controller10 then connects the call through the PSTN to the intended recipient.

[0054] Should a call be initiated within the PSTN, controller 10transmits the call information to all the cell-sites in the area. Thecell-sites in return transmit a paging message within each respectivecoverage area that is intended for the called recipient mobile user.When the intended recipient mobile unit hears the page message, itresponds with a control message that is transmitted to the nearestcell-site. This control message signals the system controller that thisparticular cell-site is in communication with the mobile unit.Controller 10 then routes the call through this cell-site to the mobileunit. Should mobile unit 16 move out of the coverage area of the initialcell-site, cell-site 12, an attempt is made to continue the call byrouting the call through another cell-site.

[0055] With respect to cellular telephone systems, The FederalCommunications Commission (FCC) has allocated a total of 25 MHz formobile-to-cell links and 25 MHz for cell-to-mobile links. The FCC hasdivided the allocation equally between two service providers, one ofwhich is the wireline telephone company for the service area and theother chosen by lottery. Because of the order in which allocations weremade, the 12.5 MHz allocated to each carrier for each direction of thelink is further subdivided into two sub-bands. For the wirelinecarriers, the sub-bands are each 10 MHz and 2.5 MHz wide. For thenon-wireline carriers, the sub-bands are each 11 MHz and 1.5 MHz wide.Thus, a signal bandwidth of less than 1.5 MHz could be fit into any ofthe sub-bands, while a bandwidth of less than 2.5 MHz could be fit intoall but one sub-band.

[0056] To preserve maximum flexibility in allocating the CDMA techniqueto the available cellular frequency spectrum, the waveform utilized inthe cellular telephone system should be less than 1.5 MHz in bandwidth.A good second choice would be a bandwidth of about 2.5 MHz, allowingfull flexibility to the wireline cellular carriers and nearly fullflexibility to non-wireline cellular carriers. While using a widerbandwidth has the advantage of offering increased multipathdiscrimination, disadvantages exist in the form of higher equipmentcosts and lower flexibility in frequency assignment within the allocatedbandwidth.

[0057] In a spread spectrum cellular telephone system, such asillustrated in FIG. 1, the preferred waveform design implementedinvolves a direct sequence pseudonoise spread spectrum carrier. The chiprate of the PN sequence is chosen to be 1.2288 MHz in the preferredembodiment. This particular chip rate is chosen so that the resultingbandwidth, about 1.25 MHz after filtering, is approximately one-tenth ofthe total bandwidth allocated to one cellular service carrier.

[0058] Another consideration in the choice of the exact chip rate isthat it is desirable that the chip rate be exactly divisible by thebaseband data rates to be used in the system. It is also desirable forthe divisor to be a power of two. In the preferred embodiment, thebaseband data rate is 9600 bits per second, leading to a choice of1.2288 MHz, 128 times 9600 for the PN chip rate.

[0059] In the cell-to-mobile link, the binary sequences used forspreading the spectrum are constructed from two different types ofsequences, each with different properties to provide differentfunctions. There is an outer code that is shared by all signals in acell or sector that is used to discriminate between multipath signals.The outer code is also used to discriminate between signals transmittedby different cells or sectors to the mobile units. There is also aninner code that is used to discriminate between user signals transmittedby single sector or cell.

[0060] The carrier waveform design in the preferred embodiment for thecell-site transmitted signals utilizes a sinusoidal carrier that isquadraphase (four phase) modulated by a pair of binary PN sequences thatprovide the outer code transmitted by a single sector or cell. Thesequences are generated by two different PN generators of the samesequence length. One sequence bi-phase modulates the in-phase channel (IChannel) of the carrier and the other sequence bi-phase modulates thequadrature phase (Q Channel) of the carrier. The resulting signals aresummed to form a composite four-phase carrier.

[0061] Although the values of a logical “zero” and a logical “one” areconventionally used to represent the binary sequences, the signalvoltages used in the modulation process are +V volts for a logical “one”and −V volts for a logical “zero”. To bi-phase modulate a sinusoidalsignal, a zero volt average value sinusoid is multiplied by the +V or −Vvoltage level as controlled by the binary sequences using a multipliercircuit. The resulting signal may then be band limited by passingthrough a bandpass filter. It is also known in the art to lowpass filterthe binary sequence stream prior to multiplying by the sinusoidalsignal, thereby interchanging the order of the operations. A quadraphasemodulator consists of two bi-phase modulators each driven by a differentsequence and with the sinusoidal signals used in the bi-phase modulatorshaving a 90° phase shift therebetween.

[0062] In the preferred embodiment, the sequence length for thetransmitted signal carrier is chosen to be 32768 chips. Sequences ofthis length can be generated by a modified maximal-length linearsequence generator by adding a zero bit to a length 32767 chip sequence.The resulting sequence has good cross-correlation and autocorrelationproperties. Good cross-correlation and autocorrelation properties arenecessary to prevent mutual interference between pilot carrierstransmitted by different cells.

[0063] A sequence this short in length is desirable in order to minimizeacquisition time of the mobile units when they first enter the systemwithout knowledge of system timing. With unknown timing, the entirelength of the sequence must be searched to determine the correct timing.The longer the sequence, the longer time the acquisition search willrequire. Although sequences shorter than 32768 could be used, it must beunderstood that as sequence length is reduced, the code processing gainis reduces. As processing gain is reduced, the rejection of multipathinterference along with interference from adjacent cells and othersources will also be reduced, perhaps to unacceptable levels. Thus,there is a desire to use the longest sequence that can be acquired in areasonable time. It is also desirable to use the same code polynomialsin all cells so that the mobile unit, not knowing what cell it is inwhen initially acquiring synchronization, can obtain fullsynchronization by searching a single code polynomial.

[0064] In order to simplify the synchronization process, all the cellsin the system are synchronized to each other. In the exemplaryembodiment, cell synchronization is accomplished by synchronizing allthe cells to a common time reference, the Navstar Global PositioningSystem satellite navigation system which is itself synchronized toUniversal Coordinated Time (UTC).

[0065] Signals from different cells are differentiated by providing timeoffsets of the basic sequences. Each cell is assigned a different timeoffset of the basic sequences differing from its neighbors. In thepreferred embodiment, the 32768 repetition period is divided into a setof 512 timing offsets. The 512 offsets are spaced 64 chips apart. Eachsector of each cell in a cellular system is also assigned a differentone of the offsets to use for all its transmissions. If there are morethan 512 sectors or cells in the system, then the offsets can be reusedin the same manner as frequencies are reused in the present analog FMcellular system. In other designs, a different number than 512 offsetscould be used. With reasonable care in assignment of pilot signaloffsets, it should never be necessary for near neighboring cells to usenear neighboring time offsets.

[0066] All signals transmitted by a cell or one of the sectors of thecell share the same outer PN codes for the I and Q channels. The signalsare also spread with an inner orthogonal code generated by using Walshfunctions. A signal addressed to a particular user is multiplied by theouter PN sequences and by a particular Walsh sequence, or sequence ofWalsh sequences, assigned by the system controller for the duration ofthe user's telephone call. The same inner code is applied to both the Iand Q channels resulting in a modulation which is effectively bi-phasefor the inner code.

[0067] It is well known in the art that a set of n orthogonal binarysequences, each of length n, for n any power of 2 can be constructed,see Digital Communications with Space Applications, S. W. Golomb et al.,Prentice-Hall, Inc., 1964, pp. 45-64. In fact, orthogonal binarysequence sets are also known for most lengths which are multiples offour and less than two hundred. One class of such sequences that is easyto generate is called the Walsh function, also known as Hadamardmatrices.

[0068] A Walsh function of order n can be defined recursively asfollows: ${W(n)} = {\begin{matrix}{{W\left( {n/2} \right)},{W\left( {n/2} \right)}} \\{{W\left( {n/2} \right)},{W^{\prime}\left( {n/2} \right)}}\end{matrix}}$

[0069] where W′ denotes the logical complement of W, and W(1)=0|.| Thus,${W(2)} = {{\begin{matrix}{0,0} \\{0,1}\end{matrix}}\quad a\quad n\quad d}$ ${W(4)} = {\begin{matrix}{0,0,0,0} \\{0,1,0,1} \\{0,0,1,1} \\{0,1,1,0}\end{matrix}}$

[0070] W(8) is as follows: ${W(8)} = {\begin{matrix}{0,0,0,0,0,0,0,0} \\{0,1,0,1,0,1,0,1} \\{0,0,1,1,0,0,1,1} \\{0,1,1,0,0,1,1,0} \\{0,0,0,0,1,1,1,1} \\{0,1,0,1,1,0,1,0} \\{0,0,1,1,1,1,0,0,} \\{0,1,1,0,1,0,0,1}\end{matrix}}$

[0071] A Walsh sequence is one of the rows of a Walsh function matrix. AWalsh function of order n contains n sequences, each of length n bits.

[0072] A Walsh function of order n (as well as other orthogonalfunctions) has the property that over the interval of n code symbols,the cross-correlation between all the different sequences within the setis zero, provided that the sequences are time aligned with each other.This can be seen by noting that every sequence differs from every othersequence in exactly half of its bits. It should also be noted that thereis always one sequence containing all zeroes and that all the othersequences contain half ones and half zeroes.

[0073] Neighboring cells and sectors can reuse the Walsh sequencesbecause the outer PN codes used in neighboring cells and sectors aredistinct. Because of the differing propagation times for signals betweena particular mobile's location and two or more different cells, it isnot possible to satisfy the condition of time alignment required forWalsh function orthogonality for both cells at one time. Thus, reliancemust be placed on the outer PN code to provide discrimination betweensignals arriving at the mobile unit from different cells. However, allthe signals transmitted by a cell are orthogonal to each other and thusdo not contribute interference to each other. This eliminates themajority of the interference in most locations, allowing a highercapacity to be obtained.

[0074] The system further envisions the voice channel to be a variablerate channel whose data rate can be varied from data block to data blockwith a minimum of overhead required to control the data rate in use. Theuse of variable data rates reduces mutual interference by eliminatingunnecessary transmissions when there is no useful speech to betransmitted. Algorithms are utilized within the vocoders for generatinga varying number of bits in each vocoder block in accordance withvariations in speech activity. During active speech, the vocoder mayproduce 20 msec. data blocks containing 20, 40, 80, or 160 bits,depending on the activity of the speaker. It is desired to transmit thedata blocks in a fixed amount of time by varying the rate oftransmission. It is further desirable not to require signaling bits toinform the receiver how many bits are being transmitted.

[0075] The blocks are further encoded by the use of a cyclic redundancycheck code (CRCC) which appends to the block an additional set of paritybits which can be used to determine whether or not the block of data hasbeen decoded correctly. CRCC check codes are produced by dividing thedata block by a predetermined binary polynomial. The CRCC consists ofall or a portion of the remainder bits of the division process. The CRCCis checked in the receiver by reproducing the same remainder andchecking to see of the received remainder bits are the same as theregenerated check bits.

[0076] In the disclosed invention, the receiving decoder decodes theblock as if it contains 160 bits, and then again as if it contains 80bits, etc. until all possible block lengths have been tried. The CRCC iscomputed for each trial decoding. If one of the trial decodings resultsin a correct CRCC, the data block is accepted and passed on to thevocoder for further processing. If no trial decoding produces a validCRCC, the received symbols are passed on to the system's signalprocessor where other processing operations can optionally be performed.

[0077] In the cell transmitter, the power of the transmitted waveform isvaried as the data rate of the block is varied. The highest data rateuses the highest carrier power. When the data rate is lower than themaximum, the modulator, in addition to lowering the power, repeats eachencoded data symbol a number of times as required to achieve the desiredtransmission rate. For example, at the lowest transmission rate, eachencoded symbol is repeated four times.

[0078] In the mobile transmitter, the peak power is held constant butthe transmitter is gated off 1/2, or 1/4 or 1/8 of the time inaccordance with the number of bits to be transmitted in the data block.The positions of the on-times of the transmitter is variedpseudo-randomly in accordance with the mobile user's addressed usercode.

[0079] Cell-to-Mobile Link

[0080] In the preferred embodiment, the Walsh function size n, is setequal to sixty-four (n=64) for the cell-to-mobile link. Therefore eachof up to sixty-four different signals to be transmitted are assigned aunique orthogonal sequence. The forward error correction (FEC) encodedsymbol stream for each voice conversation is multiplied by its assignedWalsh sequence. The Walsh coded/FEC encoded symbol stream for each voicechannel is then multiplied by the outer PN coded waveform. The.resultant spread symbol streams are then added together to form acomposite waveform.

[0081] The resulting composite waveform is then modulated onto asinusoidal carrier, bandpass filtered, translated to the desiredoperating frequency, amplified and radiated by the antenna system.Alternate embodiments of the present invention may interchange the orderof some of the just described operations for forming the cell-sitetransmitted signal. For example, it may be preferred to multiply eachvoice channel by the outer PN coded waveform and perform the filteringoperation prior to summation of all the channel signals to be radiatedby the antenna. It is well known in the art that the order of linearoperations may be interchanged to obtained various implementationadvantages and different designs.

[0082] The waveform design of the preferred embodiment for cellularservice uses the pilot carrier approach for the cell-to-mobile link asdescribed in U.S. Pat. No. 4,901,307. All cells transmit pilot carriersusing the same 32768 length sequence, but with different timing offsetsto prevent mutual interference.

[0083] The pilot waveform uses the all-zero Walsh sequence, i.e. a Walshsequence comprised of all zeroes that is found in all Walsh functionsets. The use of the all-zero Walsh sequence for all cells' pilotcarriers allows the initial search for the pilot waveform to ignore theWalsh functions until after the outer code PN synchronization has beenobtained. The Walsh framing is locked to the PN code cycle by virtue ofthe length of the Walsh frame being a factor of the PN sequence length.Therefore, provided that the cell addressing offsets of the PN code aremultiples of sixty-four chips (or the Walsh frame length) then the Walshframing is known implicitly from the outer PN code timing cycle.

[0084] All the cells in a service area are supplied with accuratesynchronization. In the preferred embodiment, a GPS receiver at eachcell synchronizes the local waveform timing to Universal CoordinatedTime (UTC). The GPS system allows time synchronization to better than 1microsecond accuracy. Accurate synchronization of cells is desirable inorder to allow easy handoff of calls between cells when mobiles movefrom one cell to another with a call in progress. If the neighboringcells are synchronized, the mobile unit will not have difficultysynchronizing to the new cell thereby facilitating a smooth handoff.

[0085] The pilot carrier is transmitted at a higher power level than atypical voice carrier so as to provide greater signal to noise andinterference margin for this signal. The higher power level pilotcarrier enables the initial acquisition search to be done at high speedand to make possible a very accurate tracking of the carrier phase ofthe pilot carrier by a relatively wide bandwidth phase tracking circuit.The carrier phase obtained from tracking the pilot carrier is used asthe carrier phase reference for demodulation of the carriers modulatedby user information signals. This technique allows many user carriers toshare the common pilot signal for carrier phase reference. For example,in a system transmitting a total of fifteen simultaneous voice carriers,the pilot carrier might be allocated a transmit power equal to fourvoice carriers.

[0086] In addition to the pilot carrier, another carrier intended to bereceived by all system users in the cell is transmitted by thecell-site. This carrier, called the synchronization channel, also usesthe same 32768 length PN sequence for spectrum spreading but with adifferent, pre-assigned Walsh sequence. The synchronization channeltransmits a broadcast message containing system information for use bythe mobiles in the system. The system information identifies thecell-site and the system and conveys information allowing the long PNcodes used for mobile information signals to be synchronized withoutadditional searching.

[0087] Another channel, called the paging channel may be provided totransmit messages to mobiles indicating that a call has arrived forthem, and to respond with channel assignments when a mobile initiates acall.

[0088] Each voice carrier transmits a digital representation of thespeech for a telephone call. The analog speech waveform is digitizedusing standard digital telephone techniques and then compressed using avocoding process to a data rate of approximately 9600 bits per second.This data signal is then rate r=1/2, constraint length K=9 convolutionalencoded, with repetition, and interleaved in order to provide errordetection and correction functions which allow the system to operate ata much lower signal-to-noise and interference ratio. Techniques forconvolutional encoding, repetition and interleaving are well known inthe art.

[0089] The resulting encoded symbols are multiplied by an assigned Walshsequence and then multiplied by the outer PN code. This process resultsin a PN sequence rate of 1.2288 MHz or 128 times the 9600 bps data rate.The resulting signal is then modulated onto an RF carrier and summedwith the pilot and setup carriers, along with the other voice carriers.Summation may be accomplished at several different points in theprocessing such as at the IF frequency, or at the baseband frequencyeither before or after multiplication by the PN sequence.

[0090] Each voice carrier is also multiplied by a value that sets itstransmitted power relative to the power of the other voice carriers.This power control feature allows power to be allocated to those linksthat require higher power due to the intended recipient being in arelatively unfavoring location. Means are provided for the mobiles toreport their received signal-to-noise ratio to allow the power to be setat a level so as to provide adequate performance without waste. Theorthogonality property of the Walsh functions is not disturbed by usingdifferent power levels for the different voice carriers provided thattime alignment is maintained.

[0091]FIG. 2 illustrates in block diagram form an exemplary embodimentcell-site equipment. At the cell-site, two receiver systems are utilizedwith each having a separate antenna and analog receiver for spacediversity reception. In each of the receiver systems the signals areprocessed identically until the signals undergoes a diversitycombination process. The elements within the dashed lines correspond toelements corresponding to the communications between the cell-site andone mobile unit. The output of the analog receivers are also provided toother elements used in communications with other mobile units.

[0092] In FIG. 2, the first receiver system is comprised of antenna 30,analog receiver 32, searcher receiver 34 and digital data receiver 36.The first receiver system may also include an optional digital datareceiver, receiver 38. The second receiver system includes antenna 40,analog receiver 42, searcher receiver 44 and digital data receiver 46.

[0093] The cell-site also includes cell-site control processor 48.Control processor 48 is coupled to data receivers 36, 38, and 46 alongwith searcher receivers 34 and 44. Control processor 48 provides amongother functions, functions such as signal processing; timing signalgeneration; power control; and control over handoff, diversity,diversity combining and system control processor interface with the MTSO(FIG. 8). Walsh sequence assignment along with transmitter and receiverassignment is also provided by control processor 48.

[0094] Both receiver systems are coupled by data receivers 36, 38, and46 to diversity combiner and decoder circuitry 50. Digital link 52 iscoupled to receive the output of diversity combiner and decodercircuitry 50. Digital link 52 is also coupled to control processor 48,cell-site transmit modulator 54 and the MTSO digital switch. Digitallink 52 is utilized to communicate signals to and from the MTSO (FIG. 8)with cell-site transmit modulator 54 and circuitry 50 under the controlof control processor 48.

[0095] The mobile unit transmitted signals are direct sequence spreadspectrum signals that are modulated by a PN sequence clocked at apredetermined rate, which in the preferred embodiment is 1.2288 MHz.This clock rate is chosen to be an integer multiple of the baseband datarate of 9.6 Kbps.

[0096] Signals received on antenna 30 are provided to analog receiver32. The details of receiver 32 are further illustrated in FIG. 3.Signals received on antenna 30 are provided to downconverter 100 whichis comprised of RF amplifier 102 and mixer 104. The received signals areprovided as an input to RF amplifier where they are amplified and outputto an input to mixer 104. Mixer 104 is provided another input, thatbeing the output from frequency synthesizer 106. The amplified RFsignals are translated in mixer 104 to an IF frequency by mixing withthe frequency synthesizer output signal.

[0097] The IF signals are then output from mixer 104 to bandpass filter(BPF) 108, typically a Surface Acoustic Wave (SAW) filter having apassband of 1.25 MHz, where they are bandpass filtered. The filteredsignals are output from BPF 108 to IF amplifier 110 where the signalsare amplified. The amplified IF signals are output from IF amplifier 110to analog to digital (A/D) converter 112 where they are digitized at a9.8304 MHz clock rate which is exactly 8 times the PN chip rate.Although A/D converter 112 is illustrated as part of receiver 32, itcould instead be a part of the data and searcher receivers. Thedigitized IF signals are output from A/D converter 112 to data receiver36, optional data receiver 38 and searcher receiver 34. The signalsoutput from receiver 32 are I and Q channel signals as discussed later.Although as illustrated in FIG. 3 with A/D converter 112 being a singledevice, with later splitting of the I and Q channel signals, it isenvisioned that channel splitting may be done prior to digitizing withtwo separate A/D converters provided for digitizing the I and Qchannels. Schemes for the RF-IF-Baseband frequency downconversion andanalog to digital conversion for I and Q channels are well known in theart.

[0098] Searcher receiver 34 is used to at the cell-site to scan the timedomain about the received signal to ensure that the associated digitaldata receiver 36, and data receiver 38 if used, are tracking andprocessing the strongest available time domain signal. Searcher receiver64 provides a signal to cell-site control processor 48 which providescontrol signals to digital data receivers 36 and 38 for selecting theappropriate received signal for processing.

[0099] The signal processing in the cell-site data receivers andsearcher receiver is different in several aspects than the signalprocessing by similar elements in the mobile unit. In the inbound, i.e.reverse or mobile-to-cell link, the mobile unit does not transmit apilot signal that can be used for coherent reference purposes in signalprocessing at the cell-site. The mobile-to-cell link is characterized bya non-coherent modulation and demodulation scheme using 64-aryorthogonal signaling.

[0100] In the 64-ary orthogonal signaling process, the mobile unittransmitted symbols are encoded into one of 2⁶, i.e. 64, differentbinary sequences. The set of sequences chosen are known as Walshfunctions. The optimum receive function for the Walsh function m-arysignal encoding is the Fast Hadamard Transform (FHT).

[0101] Referring again to FIG. 2, searcher receiver 34 and digital datareceivers 36 and 38, receive the signals output from analog receiver 32.In order to decode the spread spectrum signals transmitted to theparticular cell-site receiver through which the mobile unitcommunicates, the proper PN sequences must be generated. Further detailson the generation of the mobile unit signals are discussed later herein.

[0102] As illustrated in FIG. 3, receiver 36 includes two PN generators,PN generators 120 and 122, which generate two different short code PNsequences of the same length. These two PN sequences are common to thoseof all cell-site receivers and all mobile units with respect to theouter code of the modulation scheme as discussed in further detail laterherein. PN generators 120 and 122 thus respectively provide the outputsequences, PN_(I) and PN_(Q.) The PN_(I) and PN_(Q) sequences arerespectively referred to as the In-Phase (I) and Quadrature (Q) channelPN sequences.

[0103] The two PN sequences, PN_(I) and PN_(Q), are generated bydifferent polynomials of degree 15, augmented to produce sequences oflength 32768 rather than 32767 which would normally be produced. Forexample, the augmentation may appear in the form of the addition of asingle zero to the run of fourteen 0's in a row which appears one timein every maximal-length linear sequence of degree 15. In other words,one state of the PN generator would be repeated in the generation of thesequence. Thus the modified sequence contains one run of fifteen l's andone run of fifteen 0's. Such a PN generator circuit is disclosed in U.S.Pat. No. 5,228,054, entitled “POWER OF TWO LENGTH PSEUDO-NOISE SEQUENCEGENERATOR WITH FAST OFFSET ADJUSTMENTS”, and assigned to the assignee ofthe present invention.

[0104] In the exemplary embodiment receiver 36 also includes a long codePN generator 124 which generates a PNU sequence corresponding to a PNsequence generated by the mobile unit in the mobile-to-cell link. PNgenerator 124 can be a maximal-length linear sequence generator thatgenerates a user PN code that is very long, for example degree 42, timeshifted in accordance with an additional factor such as the mobile unitaddress or user ID to provide discrimination among users. Thus thecell-site received signal is modulated by both the long code PN_(U)sequence and the short code PN_(I) and PN_(Q) sequences. In thealternative, a non-linear encryption generator, such as an encryptorusing the data encryption standard (DES) to encrypt a 64-symbolrepresentation of universal time using a user specific key, may beutilized in place of PN generator 124.

[0105] The PN_(U) sequence output from PN generator 124 isexclusive-OR'ed with the PN_(I) and PN_(Q) sequences respectively inexclusive-OR gates 126 and 128 to provide the sequences PN_(I)′ andPN_(Q)′.

[0106] The sequences PN_(I)′ and PN_(Q)′ are provided to PN QPSKcorrelator 130 along with the I and Q channel signals output fromreceiver 32. Correlator 130 is utilized to correlate the I and Q channeldata with the PN_(I)′ and PN_(Q)′ sequences. The correlated I and Qchannel outputs of correlator 130 are respectively provided toaccumulators 132 and 134 where the symbol data is accumulated over a4-chip period. The outputs of accumulators 132 and 134 are provided asinputs to Fast Hadamard Transform (FHT) processor 136. FHT processor 148produces a set of 64 coefficients for every 6 symbols. The 64coefficients are then multiplied by a weighting function generated incontrol processor 48. The weighting function is linked to thedemodulated signal strength. The weighted data output from FHT 136 isprovided to diversity combiner and decoder circuitry 50 (FIG. 2) forfurther processing.

[0107] The second receiver system processes the received signals in amanner similar to that discussed with respect to the first receiversystem of FIGS. 2 and 3. The weighted 64 symbols output from receivers36 and 46 are provided to diversity combiner and decoder circuitry 40.Circuitry 50 includes an adder which adds the weighted 64 coefficientsfrom receiver 36 to the weighted 64 coefficients from receiver 46. Theresulting 64 coefficients are compared with one another in order todetermine the largest coefficient. The magnitude of the comparisonresult, together with the identity or the largest of the 64coefficients, is used to determine a set of decoder weights and symbolsfor use within a Viterbi algorithm decoder implemented in circuitry 50.

[0108] The Viterbi decoder contained within circuitry 50 is of a typecapable of decoding data encoded at the mobile unit with a constraintlength K=9, and of a code rate r=1/3. The Viterbi decoder is utilized todetermine the most likely information bit sequence. Periodically,nominally 1.25 msec, a signal quality estimate is obtained andtransmitted as a mobile unit power adjustment command along with data tothe mobile unit. Further information on the generation of this qualityestimate is discussed in further detail in the copending applicationmentioned above. This quality estimate is the average signal-to-noiseratio over the 1.25 msec interval.

[0109] Each data receiver tracks the timing of the received signal it isreceiving. This is accomplished by the well known technique ofcorrelating the received signal by a slightly early local reference PNand correlating the received signal with a slightly late local referencePN. The difference between these two correlations will average to zeroif there is no timing error. Conversely, if there is a timing error,then this difference will indicate the magnitude and sign of the errorand the receiver's timing is adjusted accordingly.

[0110] The cell-site further includes antenna 62 which is coupled to GPSreceiver 64. GPS receiver processes signals received on antenna 62 fromsatellites in the Navstar Global Positioning System satellite navigationsystem so as to provide timing signals indicative of UniversalCoordinated Time (UTC). GPS receiver 64 provides these timing signals tocontrol processor 48 for timing synchronizing at the cell-site asdiscussed previously.

[0111] In FIG. 2 optional digital data receiver 38 may be included forimproved performance of the system. The structure and operation of thisreceiver is similar to that described with reference to the datareceivers 36 and 46. Receiver 38 may be utilized at the cell-site toobtain additional diversity modes. This additional data receiver aloneor in combination with additional receivers can track and receive otherpossible delay paths of mobile unit transmitted signals. Optionaladditional digital data receivers such as receiver 38 providesadditional diversity modes which are extremely useful in thosecell-sites which are located in dense urban areas where manypossibilities for multipath signals occur.

[0112] Signals from the MTSO are coupled to the appropriate transmitmodulator via digital link 52 under control of control processor 48.Transmit modulator 54 under control of control processor 48 spreadspectrum modulates the data for transmission to the intended recipientmobile unit. Further details with respect to the structure and operationof transmit modulator 54 are discussed below with reference to FIGS.4a-4 c.

[0113] The output of transmit modulator 54 is provided to transmit powercontrol circuitry 56 where under the control of control processor 48 thetransmission power may be controlled. The output of circuitry 56 isprovided to summer 57 where it is summed with the output of transmitmodulator/transmit power control circuits directed to other mobiles inthe cell. The output of summer 57 is provided to transmit poweramplifier circuitry 58 where output to antenna 60 for radiating tomobile units within the cell service area. FIG. 2 further illustratespilot/control channel generators and transmit power control circuitry66. Circuitry 66 under control of control processor generates and powercontrols the pilot signal, the sync channel, and the paging channel forcoupling to circuitry 58 and output to antenna 60.

[0114] A block diagram of an exemplary embodiment of the cell-sitetransmitter is illustrated in FIGS. 4a-4 c. The transmitter includes apair of PN sequence generators used in generating the outer code. ThesePN generators generate two different PN sequences, i.e. the PN_(I) andPN_(Q) sequences, as was discussed with reference to FIG. 3. However,these PN_(I) and PN_(Q) sequences are delayed in time according to thesector or cell address.

[0115] In FIGS. 4a-4 c, the transmitter circuitry of FIG. 2 isillustrated in further detail with the pilot, sync, paging and voicechannel signals. The transmitter circuitry includes two PN generators,PN generators 196 and 198, which generate the PN_(I) and PN_(Q)sequences. PN generators 196 and 198 are responsive to an input signalcorresponding to a sector or cell address signal from the controlprocessor so as to provide a predetermined time delay to the PNsequences. These time delayed PN_(I) and PN_(Q) sequences again relaterespectively to the In-Phase (I) and Quadrature (Q) channels. Althoughonly two PN generators are illustrated for respectively generating thePN_(I) and PN_(Q) sequences for the corresponding channels of thecell-site or sector, it should be understood that many other PNgenerator schemes may be implemented. For example, in a unsectorizedcell, a pair of PN generators may be provided for each of the pilot,sync, paging and voice channels to produce, in synchronization, thePN_(I) and PN_(Q) sequences used in the outer code. Such a case may beadvantageous to avoid distributing the PN_(I) and PN_(Q) sequencesthroughout a large number of circuits.

[0116] In the preferred embodiment, Walsh function encoding of thechannel signals is employed as the inner code. In the exemplarynumerology as disclosed herein, a total of 64 different Walsh sequencesare available with three of these sequences dedicated to the pilot, syncand paging channel functions. In the sync, paging and voice channels,input data is convolutionally encoded and then interleaved as is wellknown in the art. Furthermore, the convolutional encoded data is alsoprovided with repetition before interleaving as is also well known inthe art.

[0117] The pilot channel contains no data modulation and ischaracterized as an unmodulated spread spectrum signal that all of theusers of a particular cell-site or sector use for acquisition ortracking purposes. Each cell, or if divided into sectors, each sectorhas a unique pilot signal. However, rather than using different PNgenerators for the pilot signals, it is realized that a more efficientway to generate different pilot signals is to use shifts in the samebasic sequence. Utilizing this technique a mobile unit sequentiallysearches the whole sequence and tunes to the offset or shift thatproduces the strongest correlation. In using this shift of the basicsequence, the shifts must be such that the pilots in adjacent cells orsectors must not interfere or cancel.

[0118] The pilot sequence must therefore be long enough that manydifferent sequences can be generated by shifts in the basic sequence tosupport a large number of pilot signals in the system. Furthermore, theseparation or shifts must be great enough to ensure that there is nointerference in pilot signals. Accordingly, in a exemplary embodiment ofthe present invention the pilot sequence length is chosen to be 2¹⁵. Thesequence is generated started by a sequence 2¹⁵−1 with an extra 0appended to the sequence when a particular state is detected. In theexemplary embodiment there are chosen to be 512 different pilot signalswith offsets in the basic sequence of 64 chips. However, offsets may beinteger multiples of the 64 chip offset with a corresponding reductionin the number of different pilot signals.

[0119] In generating the pilot signal, the Walsh “zero” (W₀) sequencewhich consists of all zeroes is used so as to not modulate the pilotsignal, which in essence is the PN_(I) and PN_(Q) sequences. The Walsh“zero” (W₀) sequence is therefore multiplied by the PN_(I) and PN_(Q)sequences in exclusive-OR gates. The resulting pilot signal thuscontains only the PN_(I) and PN_(Q) sequences. With all cell-sites andsectors having the same PN sequence for the pilot signal, thedistinguishing feature between cell-sites or sectors of origination ofthe transmission is the phase of the sequence.

[0120] With respect to the portion of transmit modulator and powercontrol circuitry 66 for the pilot channel, Walsh generator (W₀) 200generates a signal corresponding to the all zero function as justdiscussed. The timing in the generation of the Walsh function isprovided by the control processor, as in the case of all Walsh functiongenerators in the cell-site and mobile unit. The output of generator 200is provided as an input to both of exclusive-OR gates 202 and 204. Theother input of exclusive-OR gate 202 receives the PN_(I) signal whilethe other input of exclusive-OR gate 204 receives the PN_(Q) signal. ThePN_(I) and PN_(Q) signals are respectively exclusive-OR'ed with theoutput of generator 200 and respectively provided as inputs to FiniteImpulse Response (FIR) filters 206 and 208. The filtered signals outputfrom FIR filters 206 and 208 provided to a transmit power controlcircuitry comprised of gain control elements 210 and 212. The signalsprovided to gain control elements 210 and 212 are gain controlled inresponse to input signals (not shown) from the control processor. Thesignals output from gain control elements are provided to transmit poweramplifier circuitry 58 whose detailed structure and function isdescribed later herein.

[0121] The sync channel information is encoded and then multiplied inexclusive-OR gates by a preassigned Walsh sequence. In the exemplaryembodiment, the selected Walsh function is the (W₃₂) sequence whichconsists of a sequence of 32 “ones” followed by 32 “zeros”. Theresulting sequence is then multiplied by the PN_(I) and PN_(Q) sequencesin exclusive-OR gates.

[0122] In the exemplary embodiment the sync channel data information isprovided to the transmit modulator typically at a rate of 1200 bps. Inthe exemplary embodiment the sync channel data is preferablyconvolutionally encoded at a rate r=1/2 with a constraint length K=9,with each code symbol repeated twice. This encoding rate and constraintlength is common to all encoded forward link channels, i.e. sync, pagingand voice. In an exemplary embodiment, a shift register structure isemployed for the generators of the code G₁=753 (octal) and G₂=561(octal). The symbol rate to the sync channel is in the exemplaryembodiment 4800 sps, i.e. one symbol is 208 μsec or 256 PN chips.

[0123] The code symbols are interleaved by means of a convolutionalinterleaver spanning in the exemplary embodiment 40 msec. The tentativeparameters of the interleaver are I=16 and J=48. Further details oninterleaving is found in Data Communication, Networks and Systems,Howard W. Sams & Co., 1987, pp. 343-352. The effect of the convolutionalinterleaver is to disperse unreliable channel symbols such that any twosymbols in a contiguous sequence of I−1 or fewer symbols are separatedby at least J+1 symbols in a deinterleaver output. Equivalently, any twosymbols in a contiguous sequence of J−1 symbols are separated by atleast I+1 symbols at the deinterleaver output. In other words, if I=16and J=48, in a string of 15 symbols, the symbols are transmittedseparated by 885 μsec, thus providing time diversity.

[0124] The sync channel symbols of a particular cell or sector are tiedto the corresponding pilot signal for that cell or sector. FIG. 5illustrates the timing of two different pilot channels (N) and (N+1)which are separated by a shift of 64 chips. FIG. 5 illustrates only byway of example a timing diagram for the exemplary pilot and syncchannels with the state of the actual pilot signal chips and syncchannel symbols not illustrated. Each sync channel starts a newinterleaver cycle with the first code symbol (c_(x)) of a code symbolpair (c_(x), c′_(x)), due to a code repeat of two, shifted with respectto absolute time by an amount equal to the corresponding pilot.

[0125] As illustrated in FIG. 5, The N pilot channel starts a newinterleaver cycle, or pilot sync, at the time t_(x). Similarly, the N+1pilot channel starts a new interleaver cycle or pilot sync at the timet_(y) which occurs 64 chip later in time than time t_(x). The pilotcycle in the exemplary embodiment is 26.67 msec long, which correspondsto 128 sync channel code symbols or 32 sync channel information bits.The sync channel symbols are interleaved by a convolutional interleaverwhich spans 26.67 msec. Thus, when the mobile unit has acquired thepilot signal, it has immediate sync channel interleaver synchronization.

[0126] The sync channel symbols are covered by the preassigned Walshsequence to provide orthogonality in the signal. In the sync channel,one code symbol spans four cover sequences, i.e. one code symbol to fourrepetitions of the “32 one”-“32 zero” sequence, as illustrated in FIG.6. As illustrated in FIG. 6, a single logical “one” represents theoccurrence of 32 “one” Walsh chips while a single logical “zero”represents the occurrence of 32 “zero” Walsh chips. Orthogonality in thesync channel is still maintained even though the sync channel symbolsare skewed with respect to absolute time depending upon the associatedpilot channel because sync channel shifts are integer multiples of theWalsh frame.

[0127] The sync channel messages in the exemplary embodiment arevariable in length. The length of the message is an integer multiple of80 msec which corresponds to 3 pilot cycles. Included with the syncchannel information bits are cyclic redundancy check code (CRCC) bitsfor error detection.

[0128]FIG. 7 illustrates in the form of a timing diagram the overallexemplary system timing. In the period of two seconds there are 75 pilotcycles. In FIG. 7, the N pilot and sync channels correspond to thesector or cell using the unshifted pilot such that the pilot and syncsignals align exactly with UTC time. As such the pilot sync, i.e.initial state, aligns exactly with a common 1 pulse per second (pps)signal.

[0129] In all cases in which a shifted pilot is used, a PN phase offsetcorresponding to the pilot shift is introduced. In other words, pilotsync (initial state) and sync channel messages are skewed with respectto the 1 pps signals. The sync messages carries this phase offsetinformation so that the mobile unit can adjusts its timing accordingly.

[0130] As soon as a sync channel message has been correctly received,the mobile unit has the ability to immediately synchronize to either apaging channel or a voice channel. At pilot sync, corresponding to theend of each sync message, a new 40 msec interleaver cycle begins. Atthat time, the mobile unit starts deinterleaving the first code symbolof either a code repetition, or a (c_(x), c_(x+1)) pair, with decodersynchronization achieved. The deinterleaver write address is initializedto 0 and the read address is initialized to J, memory deinterleaversynchronization is achieved.

[0131] The sync channel messages carry information regarding the stateof a 42-bit long PN generator for the voice channel assigned for thecommunication with the mobile unit. This information is used at themobile unit digital data receivers to synchronize the corresponding PNgenerators. For example, in FIG. 7 the sync channel message N+1 containsa 42-bit field which is indicative of the state, state X, that thesector or cell voice channel corresponding long code PN generator willhave at a predetermined later time, such as 160 msec later. The mobileunit, after successfully decoding a sync channel message, loads at thecorrect instant of time the long code PN generator with the state X. Themobile unit long code PN generator is thus synchronized to permitdescrambling of the user intended messages.

[0132] With respect to the portion of transmit modulator and powercontrol circuitry 66 for the sync channel, the sync channel informationis input from the control processor to encoder 214. The sync channeldata in the exemplary embodiment is, as discussed above, convolutionalencoded by encoder 214. Encoder 214 further provides repetition of theencoded symbols, in the case of the sync channel the encoded symbols arerepeated. The symbols output from encoder 214 are provided tointerleaver 215 which provides convolutional interleaving of thesymbols. The interleaved symbols output from interleaver 215 areprovided as an input to exclusive-OR gate 216.

[0133] Walsh generator 218 generates a signal corresponding to the Walsh(W₃₂) sequence that is provided as the other input to exclusive-OR gate216. The sync channel symbol stream and the Walsh (W₃₂) sequence areexclusive-OR'ed by exclusive-OR gate 216 with the result thereofprovided as an input to both of exclusive-OR gates 220 and 222.

[0134] The other input of exclusive-OR gate 220 receives the PN_(I)signal while the other input of exclusive-OR gate 222 receives thePN_(Q) signal. The PN_(I) and PN_(Q) signals are respectivelyexclusive-OR'ed with the output of exclusive-OR gate 218 andrespectively provided as inputs to Finite Impulse Response (FIR) filters224 and 226. The filtered signals output from FIR filters 224 and 226provided to a transmit power control circuitry comprised of digitalvariable gain control elements 228 and 230. The signals provided to gaincontrol elements 228 and 230 are digitally gain controlled in responseto input digital signals (not shown) from the control processor. Thesignals output from gain control elements 228 and 230 are provided totransmit power amplifier circuitry 58.

[0135] The paging channel information is also encoded with repetition,interleaved and then multiplied by a preassigned Walsh sequence. Theresulting sequence is then multiplied by the PN_(I) and PN_(Q)sequences. The data rate of the paging channel for a particular sectoror cell is indicated in an assigned field in the sync channel message.Although the paging channel data rate is variable, it is in theexemplary embodiment fixed for each system at one of the followingexemplary data rates: 9.6, 4.8, 2.4 and 1.2 kbps.

[0136] With respect to the transmit modulator and power controlcircuitry of the paging channel, the paging channel information is inputfrom the control processor to encoder 232. Encoder 232 is in theexemplary embodiment a convolutional encoder that also providesrepetition of the symbols according to the assigned data rate of thechannel. The output of encoder 232 is provided to interleaver 233 wherethe symbols are convolutional interleaved. The output from interleaver233 is provided as an input to exclusive-OR gate 234. Although thepaging channel data rate will vary, the code symbol rate is keptconstant at 19.2 ksps by code repetition.

[0137] Walsh generator 236 generates a signal, corresponding to apreassigned Walsh sequence, that is provided as the other input toexclusive-OR gate 234. The symbol data and Walsh sequence areexclusive-OR'ed by exclusive-OR gate 234 and provided as an input toboth of exclusive-OR gates 238 and 240.

[0138] The other input of exclusive-OR gate 238 receives the PN_(I)signal while the other input of exclusive-OR gate 240 receives thePN_(Q) signal. The PN_(I) and PN_(Q) signals are respectivelyexclusive-OR'ed with the output of exclusive-OR gate 234 andrespectively provided as inputs to Finite Impulse Response (FIR) filters242 and 244. The filtered signals output from FIR filters 242 and 244are provided to a transmit power control circuitry comprised of gaincontrol elements 246 and 248. The signals provided to gain controlelements 246 and 248 are gain controlled in response to input signals(not shown) from the control processor. The signals output from gaincontrol elements are provided to transmit power amplifier circuitry 58.

[0139] The data of each voice channel is also encoded with repetition,interleaved, scrambled, multiplied by its assigned Walsh sequence(W_(i)-W_(j)), and then multiplied by the PN_(I) and PN_(Q) sequences.The Walsh sequence to be used by a particular channel is assigned by thesystem controller at call setup time in the same manner as channels areassigned to calls in the analog FM cellular system. In the exemplaryembodiment illustrated herein, up to 61 different Walsh sequences areavailable for use by the voice channels.

[0140] In the exemplary embodiment of the present invention, the voicechannel utilizes a variable data rate. The intent in using a variabledata rate is to lower the data rate when there is no voice activitythereby reducing interference generated by this particular voice channelto other users. The vocoder envisioned to provide variable rate data isdisclosed in U.S. Pat. No. 5,414,796, entitled “VARIABLE RATE VOCODER”,also assigned to the assignee of the present invention. Such a vocoderproduces data at four different data rates based on voice activity on a20 msec frame basis. Exemplary data rates are 9.6 kbps, 4.8 kbps, 2.4kbps and 1.2 kbps. Although the data rate will vary on a 20 msec basis,the code symbol rate is kept constant by code repetition at 19.2 ksps.Accordingly, the code symbols are repeated 2, 4 and 8 times for therespective data rates 4.8 kbps, 2.4 kbps and 1.2 kbps.

[0141] Since the variable rate scheme is devised to reduce interference,the code symbols at the lower rates will have lower energy. For example,for the exemplary data rates of 9.6 kbps, 4.8 kbps, 2.4 kbps and 1.2kbps, the code symbol energy (Es) is respectively E_(b)/2, E_(b)/4,E_(b)/8 and E_(b)/16 where E_(b) is the information bit energy for the9.6 kbps transmission rate.

[0142] The code symbols are interleaved by a convolutional interleaversuch that code symbols with different energy levels will be scrambled bythe operation of the interleaver. In order to keep track of what energylevel a code symbol should have a label is attached to each symbolspecifying its data rate for scaling purposes. After orthogonal Walshcovering and PN spreading, the quadrature channels are digitallyfiltered by a Finite Impulse Response (FIR) filter. The FIR filter willreceive a signal corresponding to the symbol energy level in order toaccomplish energy scaling according to the data rate. The I and Qchannels will be scaled by factors of: 1, 1/{square root}{square rootover (1/2)}, 1/2, or 1/2{square root}{square root over (2)}. In oneimplementation the vocoder would provide a data rate label in the formof a 2-bit number to the FIR filter for controlling the filter scalingcoefficient.

[0143] In FIGS. 4a-4 c, the circuitry of two exemplary voice channels,voice channels (i) and (j) are illustrated. The voice channel (i) datais input from an associated vocoder (not shown) to transmit modulator 54(FIG. 3). Transmit modulator 54 is comprised of encoder 250 _(i);interleaver 251 _(i); exclusive-OR gates 252 _(i), 255 _(i), 256 _(i)and 258 _(i); PN generator 253 _(i); and Walsh generator (W_(i)) 254_(i).

[0144] The voice channel (i) data is input to encoder 250 _(i) where inthe exemplary embodiment it is convolutional encoded with code symbolrepetition according to the input data rate. The encoded data is thenprovided to interleaver 251 _(i) where, in the exemplary embodiment, itis convolutional interleaved. Interleaver 251 _(i) also receives fromthe vocoder associated with the voice channel (i) a 2-bit data ratelabel that is interleaved with the symbol data to identify at the datarate to the FIR filters. The data rate label is not transmitted. At themobile unit, the decoder checks for all possible codes. The interleavedsymbol data is output from interleaver 251 _(i) at an exemplary rate of19.2 ksps to an input of exclusive-OR gate 255 _(i).

[0145] In the exemplary embodiment, each voice channel signal isscrambled to provide greater security in cell-to-mobile transmissions.Although such scrambling is not required it does enhance the security incommunications. For example, scrambling of the voice channel signals maybe accomplished by PN coding the voice channel signals with a PN codedetermined by the mobile unit address of user ID. Such scrambling mayuse the PN_(U) sequence or encryption scheme as discussed with referenceto FIG. 3 with respect to the particular receiver for the mobile-to-cellcommunications. Accordingly, a separate PN generator may be implementedfor this function as illustrated in FIG. 4a. Although scrambling isdiscussed with reference to a PN sequence, scrambling may beaccomplished by other techniques including those well known in the art.

[0146] Referring again to FIGS. 4a-4 c, scrambling of the voice channel(i) signal may be accomplished by providing PN generator 253 _(i) whichreceives the assigned mobile unit address from the control processor. PNgenerator 253 _(i) generates a unique PN code that is provided as theother input to exclusive-OR gate 255 _(i). The output of exclusive-ORgate 255 _(i) is provided to the one input of exclusive-OR gate 252_(i).

[0147] Walsh generator (W_(i)) 254 _(i) generates, in response to afunction select signal and timing signals from the control processor, asignal corresponding to a preassigned Walsh sequence. The value of thefunction select signal may be determined by the address of the mobileunit. The Walsh sequence signal is provided as the other input toexclusive-OR gate 252 _(i). The scrambled symbol data and Walsh sequenceare exclusive-OR'ed by exclusive-OR gate 252 _(i) with the resultprovided as an input to both of exclusive-OR gates 256 _(i) and 258_(i). PN generator 253 _(i) along with all other PN generators and Walshgenerators at the cell-site provide an output at 1.2288 MHz. It shouldbe noted that PN generator 253 includes a decimator which provides anoutput at a 19.2 kHz rate to exclusive-OR gate 255 _(i).

[0148] The other input of exclusive-OR gate 256 _(i) receives the PN_(I)signal while the other input of exclusive-OR gate 258 _(i) receives thePN_(Q) signal. The PN_(I) and PN_(Q) signals are respectivelyexclusive-OR'ed with the output of exclusive-OR gate 252 _(i) andrespectively provided as inputs to Finite Impulse Response (FIR) filters260 _(i) and 262 _(i). The input symbols are filtered according to theinput data rate label (not shown) from convolutional interleaver 251_(i). The filtered signals output from FIR filters 260 _(i) and 262 _(i)are provided to a portion of transmit power control circuitry 56comprised of gain control elements 264 _(i) and 266 _(i). The signalsprovided to gain control elements 264 _(i) and 266 _(i) are gaincontrolled in response to input signals (not shown) from the controlprocessor. The signals output from gain control elements are provided totransmit power amplifier circuitry 58.

[0149] In addition to voice bits, the forward link voice channel carriespower control information. The power control bit rate is in theexemplary embodiment 800 bps. The cell-site receiver which isdemodulating the mobile-to-cell signal from a given mobile, generatesthe power control information which is inserted in the cell-to-mobilevoice channel addressed to that particular mobile. Further details onthe power control feature is disclosed in the above identified copendingapplication.

[0150] Power control bits are inserted at the output of theconvolutional interleaver by means of a technique called code symbolpuncturing. In other words, whenever a power control bit needs to betransmitted two code symbols are replaced by two identical code symbolswith polarity given by the power control information. Moreover, powercontrol bits are transmitted at the energy level corresponding to the9600 bps bit rate.

[0151] An additional constraint imposed on the power control informationstream is that the position of the bits must be randomized amongmobile-to-cell channels. Otherwise the full energy power control bitswould generate spikes of interference at regular intervals, thusdiminishing the detectability of such bits.

[0152]FIGS. 4a-4 c further illustrate voice channel (j) which isidentical in function and structure to that of voice channel (i). It iscontemplated that there exist many more voice channels (not illustrated)with the total of voice channel being up to 61 for the illustratedembodiment.

[0153] With respect to the Walsh generators of FIGS. 4a-4 c, Walshfunctions are a set of orthogonal binary sequences that can be easilygenerated by means well known in the art. The characteristic of interestin the Walsh function is that each of the 64 sequences is perfectlyorthogonal to all of the other sequences. As such, any pair of sequencesdiffer in exactly as many bit positions as they agree, i.e. 32 over aninterval of 64 symbols. Thus when information is encoded fortransmission by the Walsh sequences the receiver will be able to selectany one of the Walsh sequences as a desired “carrier” signal. Any signalenergy encoded onto the other Walsh sequences will be rejected and notresult in mutual interference to the desired one Walsh sequence.

[0154] In the exemplary embodiment for the cell-to-mobile link, thesync, paging and voice channels as mentioned previously useconvolutional encoding of a constraint length K=9 and code rate r=1/2,that is, two encoded symbols are produced and transmitted for everyinformation bit to be transmitted. In addition to the convolutionalencoding, convolutional interleaving of symbol data is further employed.It is further envisioned that repetition is also utilized in conjunctionwith the convolutional encoding. At the mobile unit the optimum decoderfor this type of code is the soft decision Viterbi algorithm decoder. Astandard design can be used for decoding purposes. The resulting decodedinformation bits are passed to the mobile unit digital basebandequipment.

[0155] Referring again to FIGS. 4a-4 c, circuitry 58 includes series ofdigital to analog (D/A) converters for converting the digitalinformation from the PN_(I) and PN_(Q) spread data for the pilot, sync,paging and voice channels to analog form. In particular the pilotchannel PN_(I) spread data is output from gain control element 210 toD/A converter 268. The digitized data is output from D/A converter 268to an summer 284. Similarly, the output of the corresponding gaincontrol elements for the sync, paging and voice channels PN_(I) spreaddata, i.e. gain control elements 228, 246, and 264 _(i)-264 _(j), arerespectively provided to D/A converters 272, 276 and 280 i-280 j wherethe signals are digitized and provided to summer 284. The PNQ spreaddata for the pilot, sync, paging and voice channels are output from gaincontrol elements 221, 230, 248, and 266 _(i)-266 _(j), are respectivelyprovided to D/A converters 270, 274, 278 and 282 i-282 j where thesignals are digitized and provided to summer 286.

[0156] Summer 284 sums the PN_(I) spread data for the pilot, sync,paging and voice channels and while summer 286 sums the and PNQ spreaddata for the same channels. The summed I and Q channel data isrespectively input along with local oscillator (LO) frequency signalsSin(2·ft) and Cos(2·ft) to mixers 288 and 290 where they are mixed andprovided to summer 292. The LO frequency signals Sin(2·ft) and Cos(2·ft)are provided from suitable frequency sources (not shown). These mixed IFsignals are summed in summer 292 and provided to mixer 294.

[0157] Mixer 294 mixes the summed signal with an RF frequency signalprovided by frequency synthesizer 296 so as to provide frequencyupconversion to the RF frequency band. The RF signal output from mixer294 is bandpass filtered by bandpass filter 298 and output to RFamplifier 299. Amplifier 299 amplifies the band limited signal inaccordance with the input gain control signal from the transmit powercontrol circuitry 56 (FIG. 3). It should be understood that theembodiment illustrated for transmit power amplifier circuitry 58 ismerely for purposes of illustration with many variations in signalsumming, mixing, filtering and amplification possible as is well knownin the art.

[0158] Cell-site control processor 48 (FIG. 3) has the responsibilityfor assignment of digital data receivers and transmit modulators to aparticular call. Control processor 48 also monitors the progress of thecall, quality of the signals and initiates teardown on loss of signal.The cell-site communicates with the MTSO via link 52 where it is coupledby a standard telephone wire, optical fiber, or microwave link.

[0159]FIG. 8 illustrates in block diagram form the equipment utilized inthe MTSO. The MTSO typically includes a system controller or controlprocessor 300, digital switch 302, diversity combiner 304, digitalvocoder 306 and digital switch 308. Although not illustrated additionaldiversity combiners and digital vocoders are coupled between digitalswitches 302 and 308.

[0160] When the cell-diversity mode is active, the call is processed bytwo cell-sites. Accordingly, signals will arrive at the MTSO from morethan one cell-site with nominally the same information. However, becauseof fading and interference on the inbound or reverse link from themobile unit to the cell-sites, the signal from one cell-site may be ofbetter quality than the signal from the other cell-site.

[0161] Digital switch 302 is used in routing the information streamcorresponding to a given mobile unit from one or more cell-sites todiversity combiner 304 or the corresponding diversity combiner asdetermined by a signal from system control processor 300. When thesystem is not in the cell diversity mode, diversity combiner 304 may beeither bypassed or fed the same information on each input port.

[0162] A multiplicity of serial coupled diversity combiners and vocoderare provided in parallel, nominally one for each call to be processed.Diversity combiner 304 compares the signal quality indicatorsaccompanying the information bits from the two or more cell-sitesignals. Diversity combiner 304 selects the bits corresponding to thehighest quality cell-site on a frame-by-frame basis of the informationfor output to vocoder 306.

[0163] Vocoder 306 converts the format of the digitized voice signal tostandard 64 Kbps PCM telephone format, analog, or any other standardformat. The resultant signals is transmitted from vocoder 306 to digitalswitch 308. Under the control of system control processor 300, the callis routed to the PSTN.

[0164] Voice signals coming from the PSTN intended for the mobile units,are provided to digital switch 308 for coupling to an appropriatedigital vocoder such as vocoder 306 under control of system controlprocessor 300. Vocoder 306 encodes the input digitized voice signals andprovides the resulting information bit stream directly to digital switch302. Digital switch 302 under system control processor control directthe encoded data to the cell-site or cell-sites to which the mobile unitis communicating. Although discussed previously that informationtransmitted to the MTSO analog voice, it is further envisioned thatdigital information may also be communicated in the system. To ensurecompatibility with the system, care must be taken in proper framing ofthe data.

[0165] If the mobile unit is in a handoff mode communicating to multiplecell-sites or in a cell diversity mode, digital switch 302 routes thecalls to the appropriate cell-sites for transmission by the appropriatecell-site transmitter to the intended recipient mobile unit. However, ifthe mobile unit is communicating with only a single cell-site or not ina cell diversity mode, the signal is directed only to a singlecell-site.

[0166] System control processor 300 provides control over digitalswitches 302 and 306 for routing data to and from the MTSO. Systemcontrol processor 300 also determines the assignment of calls to thecell-sites and to the vocoders at the MTSO. Furthermore, system controlprocessor 300 communicates with each cell-site control processor aboutthe assignment of particular calls between the MTSO and cell-site, andthe assignment of PN codes for the calls. It should be furtherunderstood that as illustrated in FIG. 8 digital switches 302 and 306are illustrated as two separate switches, however, this function may beperformed by a single physical switching unit.

[0167] When the cell-diversity mode is in use, the mobile unit will usethe searcher receiver to identify and acquire the strongest multipathsignal from each of the two cell-sites. The digital data receivers willbe controlled by the searcher receiver and the control processor so asto demodulate the strongest signals. When the number of receivers isless than the number of cell-sites transmitting information in parallel,a switching diversity capability is possible. For example, with only asingle data receiver and with two cell-sites transmitting, the searcherwill monitor the pilots from both cell-sites and choose the strongestsignal for the receiver to demodulate. In this embodiment the choice canbe made as frequently as every vocoder frame, or about every 20 msec.

[0168] The system control processor has responsibility for assignment ofdigital data receivers and modulators at the cell-site to handleparticular calls. Thus in the cell-to-mobile link, the system controlprocessor controls the assignment of Walsh sequences used at thecell-site in transmission of a particular call to the mobile unit. Inaddition the system control processor controls the receiver Walshsequences and PN codes. In the mobile-to-cell link, the system controlprocessor also controls the mobile unit user PN codes for the call.Assignment information is therefore transmitted from the MTSO to thecell-site and from there to the cell to the mobile. The system controlprocessor also monitors the progress of the call, the quality ofsignals, and initiates tear down on loss of signal.

[0169] Mobile-to Cell Link

[0170] In the mobile-to-cell link, the channel characteristics dictatethat the modulation technique be modified. In particular, the use of apilot carrier as is used in the cell-to-mobile link is no longerfeasible. The pilot carrier must be more powerful than a voice carrierin order to provide a good phase reference for data modulation. With thecell-site transmitting many simultaneous voice carriers, a single pilotsignal can be shared by all the voice carriers. Therefore, the pilotsignal power per voice carrier is quite small.

[0171] In the mobile-to-cell link, however, there is usually only asingle voice carrier per mobile. If a pilot were used, it would requiresignificantly more power than the voice carrier. This situation isclearly not desirable since overall system capacity would be greatlyreduced due to the interference caused by the presence of a largernumber of high power pilot signals. Therefore, a modulation capable ofefficient demodulation without a pilot signal must be used.

[0172] With the mobile-to-cell channel corrupted by Rayleigh fading,resulting in a rapidly varying channel phase, coherent demodulatortechniques, such as a Costas loop which derives phase from the receivedsignal, are not feasible. Other techniques such as differentiallycoherent PSK can be employed but fail to provide the desired level ofsignal-to-noise ratio performance.

[0173] Thus, a form of orthogonal signaling such as binary, quaternaryor m-ary signaling should be employed. In the exemplary embodiment, a64-ary orthogonal signaling technique is employed using Walsh functions.The demodulator for m-ary orthogonal signaling requires channelcoherence only over the duration of transmission of the m-ary symbol. Inthe exemplary embodiment, this is only two bit times.

[0174] The message encoding and modulation process begins with aconvolutional encoder of constraint length K=9 and code rate r=1/3. At anominal data rate of 9600 bits per second, the encoder produces 28800binary symbols per second. These are grouped into characters containing6 symbols each at a rate of 4800 characters per second with there being64 possible characters. Each character is encoded into a length 64 Walshsequence containing 64 binary bits or “chips.” The 64-ary Walsh chiprate is 307,200 chips per second in the exemplary embodiment.

[0175] The Walsh chips are then “covered” or multiplied by a PN sequencerunning at the rate of 1.2288 MHz. Each mobile unit is assigned a uniquePN sequence for this purpose. This PN sequence can either be assignedonly for the duration of the call or assigned permanently to the mobileunit. The assigned PN sequence is referred to herein as the user PNsequence. The user PN sequence generator runs at a clock rate of 1.2288MHz and so as to produce four PN chips for every Walsh chip.

[0176] Finally, a pair of short, length 32768, PN sequences aregenerated. In the exemplary embodiment, the same sequences are used asfor the cell-to-mobile link. The user PN sequence covered Walsh chipsequence is then covered or multiplied by each of the two short PNsequences. The two resulting sequences then bi-phase modulate aquadrature pair of sinusoids and are summed into a single signal. Theresulting signal is then bandpass filtered, translated to the final RFfrequency, amplified, filtered and radiated by the antenna of the mobileunit. As was discussed with reference to the cell-to-mobile signal, theordering of the filtering, amplification, translation and modulationoperations may be interchanged.

[0177] In an alternative embodiment, two different phases of the user PNcode might be produced and used to modulate the two carrier phases ofthe quadraphase waveform, dispensing with the need for using the length32768 sequences. In yet another alternative, the mobile-to-cell linkmight utilize only bi-phase modulation, also dispensing with the needfor the short sequences.

[0178] The cell-site receiver for each signal produces the short PNsequences and the user PN sequence for each active mobile signal beingreceived. The receiver correlates the received signal energy with eachof the coded waveforms in separate correlators. Each of the correlatoroutputs is then separately processed to demodulate the 64-ary encodingand the convolutional coding using a Fast Hadamard Transform processorand a Viterbi algorithm decoder.

[0179] In another alternative modulation scheme for the mobile-to-celllink, the same modulation scheme would be used as for the cell-to-mobilelink. Each mobile would utilize the pair of 32768 length sector codes asouter codes. The inner code would utilize a length 64 Walsh sequencethat is assigned to the mobile for use while it is in that sector.Nominally, the same Walsh sequence would be assigned to the mobile forthe mobile-to-cell link as is used for the cell-to-mobile link.

[0180] The above orthogonal PN coding scheme limits the availablebandwidth spreading that can be used by the modulation system to amaximum rate of the chip rate divided by 64, or 19200 Hz for the numbersused in the exemplary embodiment. This would preclude the use of m-aryencoding with large m as described for the exemplary embodiment. As analternative, however, a rate r=1/2, constraint length K=9 convolutionalcode could be used with differential binary phase shift keyingmodulation of the encoded binary symbols. The demodulator in thecell-site could build up a phase reference over a short interval usingthe technique described in the article “Nonlinear Estimation ofPSK-Modulated Carrier with Application to Burst Digital Transmission”,Andrew J. Viterbi and Audrey M. Viterbi, IEEE Transactions OnInformation Theory, Vol IT-29, No. 4, July 1983. For example, a phasereference could be averaged over only 4 symbols requiring no morechannel coherence than the above 64-ary scheme.

[0181] The performance of the just described alternative scheme,however, will be inferior to the preferred embodiment in the presence ofsevere Rayleigh fading and multipath conditions. However, in certainenvironments where fading and multipath are less severe, for example,the satellite-mobile channel and in certain land-mobile channels, theperformance of the alternative system could be better than the preferredembodiment. This can occur because the gain from making the mobilesignals orthogonal to each other may exceed the loss in detectionefficiency of the DPSK scheme.

[0182] In order to satisfy the requirement for time alignment inorthogonal Walsh functions for the alternative mobile-to-cell link, eachcell receiver determines the time error from nominal timing of eachreceived signal. If a given received signal lags in timing, then theassociated cell modulator and transmitter will transmit a command tothis mobile to advance its transmit timing by a small increment.Conversely, if the received signal timing of a mobile leads the nominaltiming, a command to retard by a small increment is transmitted to themobile. The timing adjustment increments are made on the order of 1/8 PNchip or 101.7 nanoseconds. The commands are transmitted at a relativelylow rate, on the order of 10 to 50 Hz and consist of a single bitinserted into the digital voice data flow.

[0183] During a soft handoff operation, the mobile unit will bereceiving signals from two or more cells. Because the mobile unit canonly align its timing in response to one of cells' timing adjustcommands, the mobile unit will normally move its timing in response tothe commands received from the strongest cell being received. The mobileunit transmitted signal will thus be in time alignment with the cellwith which it has the best path. Otherwise greater mutual interferenceto other users will result.

[0184] If each cell receiver receiving a mobile signal performs theabove time error measurement and correction transmission operation, thenall the mobiles' received signals will normally be received withapproximately the same timing, resulting in reduced interference.

[0185]FIG. 9 illustrates in block diagram form an exemplary mobile unitCDMA telephone set. The mobile unit CDMA telephone set includes anantenna 430 which is coupled through diplexer 432 to analog receiver 344and transmit power amplifier 436. Antenna 430 and diplexer 432 are ofstandard design and permit simultaneous transmission and receptionthrough a single antenna. Antenna 430 collects transmitted signals andprovides them through diplexer 432 to analog receiver 434. Receiver 434receives the RF frequency signals from diplexer 432 which are typicallyin the 850 MHz frequency band for amplification and frequencydownconversion to an IF frequency. This translation process isaccomplished using a frequency synthesizer of standard design whichpermits the receiver to be tuned to any of the frequencies within thereceive frequency band of the overall cellular telephone frequency band.The signals are also filtered and digitized for providing to digitaldata receivers 540 and 542 along with searcher receiver 544.

[0186] The details of receiver 434 are further illustrated in FIG. 10.Received signals from antenna 430 are provided to downconverter 500which is comprised of RF amplifier 502 and mixer 504. The receivedsignals are provided as an input to RF amplifier 502 where they areamplified and output as an input to mixer 504. Mixer 504 is providedwith another input, that being the signal output from frequencysynthesizer 506. The amplified RF signals are translated in mixer 504 toan IF frequency by mixing with the frequency synthesizer output signal.

[0187] The IF signals are output from mixer 504 to bandpass filter (BPF)508, typically a Surface Acoustic Wave (SAW) filter having a passband ofapproximately 1.25 MHz, where they are from bandpass filtered. Thecharacteristics of the SAW filter are chosen to match the waveform ofthe signal transmitted by the cell-site. The cell-site transmittedsignal is a direct sequence spread spectrum signal that is modulated bya PN sequence clocked at a predetermined rate, which in the exemplaryembodiment is 1.2288 MHz. This clock rate is chosen to be an integermultiple of the baseband data rate of 9.6 kbps.

[0188] The filtered signals are output from BPF 508 as an input to avariable gain IF amplifier 510 where the signals are again amplified.The amplified IF signals are output from IF amplifier 510 to analog todigital (A/D) converter 512 where the signals are digitized. Theconversion of the IF signal to a digital signal occurs at a 9.8304 MHzclock rate in the exemplary embodiment which is exactly eight times thePN chip rate. Although A/D converter 512 is illustrated as part ofreceiver 534, it could instead be a part of the data and searcherreceivers. The digitized IF signals are output from A/D converter 512 todata receivers 440 and 442, and searcher receiver 444.

[0189] Receiver 434 also performs a power control function for adjustingthe transmit power of the mobile unit. An automatic gain control (AGC)circuit 514 is also coupled to the output of IF amplifier 510. Inresponse to the level of the amplified IF signal, AGC circuit 514provides a feedback signal to the gain control input of IF amplifier510. Receiver 434 also uses AGC circuit 514 to generate an analog powercontrol signal that is provided to transmit power control circuitry 438.

[0190] In FIG. 9, the digitized signal output from receiver 434 isprovided to digital data receivers 440 and 442 and to searcher receiver444. It should be understood that an inexpensive, low performance mobileunit might have only a single data receiver while higher performanceunits may have two or more to allow diversity reception.

[0191] The digitized IF signal may contain the signals of many on-goingcalls together with the pilot carriers transmitted by the currentcell-site and all neighboring cell-sites. The function of the receivers440 and 442 are to correlate the IF samples with the proper PN sequence.This correlation process provides a property that is well-known in theart as “processing gain” which enhances the signal-to-interference ratioof a signal matching the proper PN sequence while not enhancing othersignals. Correlation output is then synchronously detected using thepilot carrier from the closest cell-site as a carrier phase reference.The result of this detection process is a sequence of encoded datasymbols.

[0192] A property of the PN sequence as used in the present invention isthat discrimination is provided against multipath signals. When thesignal arrives at the mobile receiver after passing through more thanone path, there will be a difference in the reception time of thesignal. This reception time difference corresponds to the difference indistance divided by the velocity of propagation. If this time differenceexceeds one microsecond, then the correlation process will discriminatebetween the paths. The receiver can choose whether to track and receivethe earlier or later path. If two receivers are provided, such asreceivers 440 and 442, then two independent paths can be tracked andprocessed in parallel.

[0193] Searcher receiver 444, under control of control processor 446 isfor continuously scanning the time domain around the nominal time of areceived pilot signal of the cell-site for other multi-path pilotsignals from the same cell-site and for other cell-site transmittedpilot signals. Receiver 444 will measure the strength of any receptionof a desired waveform at times other than the nominal time. Receiver 444compares signal strength in the received signals. Receiver 444 providesa signal strength signal to control processor 446 indicative of thestrongest signals.

[0194] Processor 446 provides control signals to data receivers 440 and442 for each to process a different one of the strongest signals. Onoccasion another cell-site transmitted pilot signal is of greater signalstrength than the current cell-site signal strength. Control processor446 then would generate a control message for transmission to the systemcontroller via the current cell-site requesting a transfer of the cellto the cell-site corresponding to the strongest pilot signal. Receivers440 and 442 may therefore handle calls through two different cell-sites.

[0195] During a soft handoff operation, the mobile unit will bereceiving signals from two or more cells. Because the mobile unit canonly align its timing in response to one of cells' timing adjustcommands, the mobile unit will normally move its timing in response tothe commands received from the strongest cell being received. The mobileunit transmitted signal will thus be in time alignment with the cellwith which it has the best path. Otherwise greater mutual interferenceto other users will result.

[0196] Further details of an exemplary receiver, such as data receiver440 is illustrated in further detail in FIG. 10. Data receiver 440includes PN generators 516 and 518 which generate the PN_(I) and PN_(Q)sequences in a manner and corresponding to those generated by thecell-site. Timing and sequence control signals are provided to PNgenerators 516 and 518 from control processor 446. Data receiver 440also includes Walsh generator 520 which provides the appropriate Walshfunction for communication with this mobile unit by the cell-site. Walshgenerator 520 generates, in response to timing signals (not shown) and afunction select signal from the control processor, a signalcorresponding to an assigned Walsh sequence. The function select signalis transmitted to the mobile unit by the cell-site as part of the callset up message. The PN_(I) and PN_(Q) sequences output from PNgenerators 516 and 518 are respectively input to exclusive-OR gates 522and 524. Walsh generator 520 provides its output to both of exclusive-ORgates 522 and 524 where the signals are exclusive-OR'ed and output thesequences PN_(I)′ and PN_(Q)′.

[0197] The sequences PN_(I)′ and PN_(Q)′ are provided to receiver 440where they are input to PN QPSK correlator 526. PN correlator 526 may beconstructed in a manner similar to the PN correlator of the cell-sitedigital receivers. PN correlator 526 correlates the received I and Qchannel data with the PN_(I)′ and PN_(Q)′ sequences and providescorrelated I and Q channel data output to corresponding accumulators 528and 530. Accumulators 528 and 530 accumulate the input information overa period of one symbol or 64 chips. The accumulator outputs are providedto phase rotator 532 which also receives a pilot phase signal fromcontrol processor 446. The phase of the received symbol data is rotatedin accordance with the phase of the pilot signal as determined by thesearcher receiver and the control processor. The output from phaserotator 532 is the I channel data which is provided to the deinterleaverand decoder circuitry.

[0198] Control processor 446 also includes PN generator 534 whichgenerates the user PN sequence in response to an input mobile unitaddress or user ID. The PN sequence output from PN generator 534 isprovided to diversity combiner and decoder circuitry. Since thecell-to-mobile signal is scrambled with the mobile user address PNsequence, the output from PN generator 534 is used in descrambling thecell-site transmitted signal intended for this mobile user similar tothat as in the cell-site receiver. PN generator 534 specificallyprovides the output PN sequence to the deinterleaver and decodercircuitry where it is used to descramble the scrambled user data.Although scrambling is discussed with reference to a PN sequence, it isenvisioned that other scrambling techniques including those well knownin the art may be utilized.

[0199] The outputs of receivers 440 and 442 are thus provided todiversity combiner and decoder circuitry 448. The diversity combinercircuitry contained within circuitry 448 simply adjusts the timing ofthe two streams of received symbols into alignment and adds themtogether. This addition process may be proceeded by multiplying the twostreams by a number corresponding to the relative signal strengths ofthe two streams. This operation can be considered a maximal ratiodiversity combiner. The resulting combined signal stream is then decodedusing a forward error detection (FEC) decoder also contained withincircuitry 448. The usual digital baseband equipment is a digital vocodersystem. The CDMA system is designed to accommodate a variety ofdifferent vocoder designs.

[0200] Baseband circuitry 450 typically includes a digital vocoder (notshown) which may be a variable rate type as disclosed in the previouslymentioned copending patent application. Baseband circuitry 450 furtherserves as an interface with a handset or any other type of peripheraldevice. Baseband circuitry 450 accommodates a variety of differentvocoder designs. Baseband circuitry 450 provides output informationsignals to the user in accordance with the information provided theretofrom circuitry 448.

[0201] In the mobile-to-cell link, user analog voice signals aretypically provided through a handset as an input to baseband circuitry450. Baseband circuitry 450 includes an analog to digital (A/D)converter (not shown) which converts the analog signal to digital form.The digital signal is provided to the digital vocoder where it isencoded. The vocoder output is provided to a forward error correction(FEC) encoding circuit (not shown) for error correction. In theexemplary embodiment the error correction encoding implemented is of aconvolutional encoding scheme. The digitized encoded signal is outputfrom baseband circuitry 450 to transmit modulator 452.

[0202] Transmit modulator 452 first Walsh encodes the transmit data andthen modulates the encoded signal on a PN carrier signal whose PNsequence is chosen according to the assigned address function for thecall. The PN sequence is determined by control processor 446 from callsetup information that is transmitted by the cell-site and decoded byreceivers 440 and 442 and control processor 446. In the alternative,control processor 446 may determine the PN sequence throughprearrangement with the cell-site. Control processor 446 provides the PNsequence information to transmit modulator 452 and to receivers 440 and442 for call decoding.

[0203] The output of transmit modulator 452 is provided to transmitpower control circuitry 438. Signal transmission power is controlled bythe analog power control signal provided from receiver 434. Control bitstransmitted by the cell-sites in the form of power adjustment commandsare processed by data receivers 440 and 442. The power adjustmentcommands are used by control processor 446 in setting the power level inmobile unit transmission. In response to these commands, controlprocessor 446 generates a digital power control signal that is providedto circuitry 438. Further information on the relationship of receivers440 and 442, control processor 446 and transmit power control 438 withrespect to power control is further described in the above-mentionedU.S. Pat. No. 5,056,109.

[0204] Transmit power control circuitry 438 outputs the power controlledmodulated signal to transmit power amplifier circuitry 436. Circuitry436 amplifies and converts the IF signal to an RF frequency by mixingwith a frequency synthesizer output signal which tunes the signal to theproper output frequency. Circuitry 436 includes an amplifier whichamplifies the power to a final output level. The intended transmissionsignal is output from circuitry 436 to diplexer 432. Diplexer 432couples the signal to antenna 340 for transmission to the cell-sites.

[0205] Control processor 446 also is capable of generating controlmessages such as cell-diversity mode requests and cell-sitecommunication termination commands. These commands are provided totransmit modulator 452 for transmission. Control processor 446 isresponsive to the data received from data receivers 440 and 442, andsearch receiver 444 for making decisions relative to handoff anddiversity combining.

[0206] With respect to transmission by the mobile unit, the mobile useranalog voice signal is first passed through a digital vocoder. Thevocoder output is then, in sequence, convolutional forward errorcorrection (FEC) encoded, 64-ary orthogonal sequence encoded andmodulated on a PN carrier signal. The 64-ary orthogonal sequence isgenerated by a Walsh function encoder. The encoder is controlled bycollecting six successive binary symbol outputs from the convolutionalFEC encoder. The six binary collectively determine which of the 64possible Walsh sequences will be transmitted. The Walsh sequence is 64bits long. Thus, the Walsh “chip” rate must be 9600*3*(1/6)*64=307200 Hzfor a 9600 bps data transmission rate.

[0207] In the mobile-to-cell link, a common short PN sequence is usedfor all voice carriers in the system, while user address encoding isdone using the user PN sequence generator. The user PN sequence isuniquely assigned to the mobile for at least the duration of the call.The user PN sequence is exclusive-OR'ed with the common PN sequences,which are length 32768 augmented maximal-length linear shift registersequences. The resulting binary signals then each bi-phase modulate aquadrature carrier, are summed to form a composite signal, are bandpassfiltered, and translated to an IF frequency output. In the exemplaryembodiment, a portion of the filtering process is actually carried outby a finite impulse response (FIR) digital filter operating on thebinary sequence output.

[0208] The modulator output is then power controlled by signals from thedigital control processor and the analog receiver, converted to the RFfrequency of operation by mixing with a frequency synthesizer whichtunes the signal to proper output frequency, and then amplified to thefinal output level. The transmit signal is then passed on to thediplexer and the antenna.

[0209]FIG. 11 illustrates a preferred, but yet exemplary, embodiment ofmobile unit transmit modulator 452. Data is provided in digital formfrom the user digital baseband circuitry to encoder 600 where in theexemplary embodiment is convolutionally encoded. The output of encoder600 is provided to interleaver 602 which in the exemplary embodiment isa block interleaver. The interleaved symbols are output from blockinterleaver 602 to Walsh encoder 604 of transmit modulator 452. Walshencoder 604 utilizes the input symbols to generate a code sequenceoutput. The Walsh sequence is provided to one input of exclusive-OR gate606.

[0210] Transmit modulator 452 further includes PN generator 608 whichreceives the mobile unit address as an input in determining the outputPN sequence. PN generator 608 generates the user specific 42-bitsequence as was discussed with reference to FIGS. 3 and 4a-4 c. Afurther attribute of PN generator 608 that is common to all user PNgenerators and not previously discussed is the use of a maskingtechnique in generating the output user PN sequence. For example, a42-bit mask is provided for that user with each bit of the 42-bit maskexclusive-OR'ed with a bit output from each register of the series ofshift register that form the PN generator. The results of the mask andshift register bit exclusive-OR operation are then exclusive-OR'edtogether to form the PN generator output that is used as the user PNsequence. The output PN sequence of PN generator 608, the sequencePN_(U), is input to exclusive-OR gate 606. The Walsh symbol data and thePN_(U) sequence are exclusive-OR'ed in exclusive-OR gate 606 andprovided as in input to both of exclusive-OR gates 610 and 612.

[0211] Transmit modulator 452 further includes PN generators 614 and 616which respectively generate PN_(I) and PN_(Q) sequences. All mobileunits use the same PN_(I) and PN_(Q) sequences. These PN sequences arein the exemplary embodiment the zero-shift used in the cell-to-mobilecommunications. The other input of exclusive-OR gates 610 and 612 arerespectively provided with the PN_(I) and PN_(Q) sequences output fromPN generators 614 and 616. The sequences PN_(I) and PN_(Q) areexclusive-OR'ed in the respective exclusive-OR gates with the outputprovided to transmit power control 438 (FIG. 9).

[0212] In the exemplary embodiment, the mobile-to-cell link uses rater=1/3 convolutional code with constraint length K=9. The generators forthe code are G₁=557 (octal), G₂=663 (octal), and G₃=711 (octal). Similarto the cell-to-mobile link, code repetition is used to accommodate thefour different data rates that the vocoder produces on a 20 msec framebasis. Unlike the cell-to-mobile link, the repeated code symbols are nottransmitted over the air at lower energy levels, rather only one codesymbol of a repetition group is transmitted at the nominal power level.In conclusion, the code repetition in the exemplary embodiment is usedmerely as an expedient to fit the variable data rate scheme in theinterleaving and modulation structure as it will be shown in thefollowing paragraphs.

[0213] A block interleaver spanning 20 msec, exactly one vocoder frame,is used in the mobile-to-cell link. The number of code symbols in 20msec, assuming a data rate of 9600 bps and a code rate r=1/3, is 576.The N and B parameters, N is equal to the number of rows and B to thenumber of columns of the interleaver array are 32 and 18, respectively.The code symbols are written into the interleaver memory array by rowsand read out by columns.

[0214] The modulation format is 64-ary orthogonal signaling. In otherwords, interleaved code symbols are grouped into groups of six to selectone out of 64 orthogonal waveforms. The 64 time orthogonal waveforms arethe same Walsh functions used as cover sequences in the cell-to-mobilelink.

[0215] The data modulation time interval is equal to 208.33 μsec, and isreferred to as a Walsh symbol interval. At 9600 bps, 208.33 μseccorresponds to 2 information bits and equivalently to 6 code symbols ata code symbol rate equal to 28800 sps. The Walsh symbol interval issubdivided into 64 equal length time intervals, referred to as Walshchips, each lasting 208.33/64=3.25 μsec. The Walsh chip rate is then1/3.25 μsec=307.2 kHz. Since the PN spreading rate is symmetric in thetwo links, i.e. 1.2288 MHz, there are exactly 4 PN chips per Walsh chip.

[0216] A total of three PN generators are used in the mobile-to-celllink path. The user specific 42-bit PN generator and the pair of 15-bitI and Q channel PN generators. Following the user specific spreadingoperation, the signal is QPSK spread as it was done in thecell-to-mobile link. Unlike the cell-to-mobile link, where each sectoror cell was identified by unique sequences of length 2¹⁵, here allmobile units use the same I and Q PN sequences. These PN sequences arethe zero-shift sequences used in the cell-to-mobile link, also referredto as the pilot sequences.

[0217] Code repetition and energy scaling are used in the cell-to-mobilelink to accommodate the variable rates produced by the vocoder. Themobile-to-cell link uses a different scheme based on a bursttransmission.

[0218] The vocoder produces four different data rates, i.e. 9600, 4800,2400, and 1200 bps, on a 20 msec frame basis as in the cell-to-mobilelink. The information bits are encoded by the rate r=1/3 convolutionalencoder and code symbols are repeated 2, 4, and 8 times at the threelower data rates. Thus, the code symbol rate is kept constant at 28800sps. Following the encoder, the code symbols are interleaved by theblock interleaver which spans exactly one vocoder frame or 20 msec. Atotal of 576 code symbols are generated every 20 msec by theconvolutional encoder, some of which might be repeated symbols.

[0219] The code symbols sequence as it is transmitted is shown in FIG.12. Notice that a vocoder frame, 20 msec, has been subdivided into 16slots each lasting 1.25 msec. The numerology of the mobile-to-cell linkis such that in each slot there are 36 code symbols at the 28800 spsrate or equivalently 6 Walsh symbols at the 4800 sps rate. At the 1/2rate, i.e. 4800 bps, the slots are grouped into 8 groups each comprising2 slots. At the 1/4 rate, i.e. 2400 bps, the slots are grouped into 4groups each comprising 4 slots, and finally at the 1/8 rate, i.e. 1200bps, the slots are grouped into 2 groups each comprising 8 slots.

[0220] An exemplary symbol burst transmission pattern is furtherillustrated in FIG. 12. For example, at the 1/4 rate, i.e. 2400 bps,during the fourth slot of the first group the fourth and eighth row ofthe interleaver memory array are read out by columns and sequentiallytransmitted. The slot position for the transmitted data must berandomized in order to reduce the interference.

[0221] The mobile-to-cell link timing is illustrated in FIG. 13. FIG. 13expands upon the timing diagram of FIG. 7 to include the mobile-to-cellchannels, i.e. voice and access. The synchronization of themobile-to-cell link comprises the following steps:

[0222] 1. Decode successfully a sync message, i.e. CRC check;

[0223] 2. Load long PN shift register with state received in the syncmessage; and

[0224] 3. Compensate for pilot code phase offset if receiving from asector which uses a shifted pilot.

[0225] At this point the mobile has complete synchronization, i.e. PNsynchronization and real time synchronization, and can begin to transmiton either the access channel or voice channel.

[0226] The mobile unit in order to originate a call must be providedwith signaling attributes in order to complete a call to another systemuser via a cell-site. In the mobile-to-cell link the envisioned accesstechnique is the slotted ALOHA. An exemplary transmission bit rate onthe reverse channel is 4800 bps. An access channel packet comprises of apreamble followed by the information.

[0227] The preamble length is in the exemplary embodiment an integermultiple of 20 msec frames and is a sector/cell parameter which themobile receives in one of the paging channel messages. Since the cellreceivers use the preambles to resolve propagation delays this schemeallows the preamble length to vary based on the cell radius. The usersPN code for the access channel is either prearranged or transmitted tothe mobile units on the paging channel.

[0228] The modulation is fixed and constant for the duration of thepreamble. The orthogonal waveform used in the preamble is W₀, i.e. theall zero Walsh function. Notice that an all zero pattern at the input ofthe convolutional encoder generates the desired waveform W₀.

[0229] An access channel data packet may consist of one or at most two20 msec frames. The coding, interleaving, and modulation of the accesschannel is exactly the same as for a voice channel at the 9600 bps rate.In an exemplary embodiment, the sector/cell requires the mobile units totransmit a 40 msec preamble and the access channel message type requiresone data frame. Let N_(P) be the number of preamble frames where k isthe number of 20 msec elapsed from a predefined time origin. Thenmobiles are allowed to initiate transmission on the access channel onlywhen the equation: (k, N_(P)+2)=0 is true.

[0230] With respect to other communications applications it may bedesirable to rearrange the various elements of the error correctioncoding, the orthogonal sequence coding and the PN coding to better fitthe application.

[0231] For example, in satellite mobile communications where the signalsare relayed between large Hub earth stations and the mobile terminals byone or more earth orbiting satellites, it may be desirable to employcoherent modulation and demodulation techniques in both directions ofthe link because the channel is much more phase coherent than theterrestrial mobile channel. In such an application, the mobile modulatorwould not utilize m-ary encoding as described above. Instead, bi-phaseor four-phase modulation of forward error correction symbols might beemployed with conventional coherent demodulation with carrier phaseextracted from the received signal using Costas loop techniques. Inaddition, the orthogonal Walsh function channelization such as hereindescribed for the cell-to-mobile link may be employed. As long as thechannel phase remains reasonably coherent, this modulation anddemodulation system provides operation with lower Eb/No than m-aryorthogonal signaling resulting in higher system capacity.

[0232] In another embodiment, it may be preferable to encode the speechwaveform directly into the RF waveform instead of utilizing a vocoderand FEC techniques. While the use of a vocoder and FEC techniques resultin very high link performance, the complexity of implementation is high,resulting in additional cost and in high power consumption. Thesedisadvantages may be especially unfavorable in a pocket portabletelephone where battery consumption and cost are important. In customarydigital telephone transmission practice, the speech waveform isrepresented in a digital format as 8 bit speech samples at a sample rateof 8 kHz. The CDMA system could encode the 8 bit samples directly intocarrier phase angles. This would eliminate the need for a vocoder or aFEC encoder/decoder. It would also require a somewhat highersignal-to-noise ratio for good performance, resulting in lower capacity.In another alternative, the 8 bit speech samples could be directlyencoded into carrier amplitudes. In yet another alternative, the speechwaveform samples could be encoded into carrier phases and amplitudes.

[0233] The previous description of the preferred embodiments is providedto enable any person skilled in the art to make or use the presentinvention. The various modifications to these embodiments will bereadily apparent to those skilled in the art, and the generic principlesdefined herein may be applied to other embodiments without the use ofthe inventive faculty. Thus, the present invention is not intended to belimited to the embodiments shown herein but is to be accorded the widestscope consistent with the principles and novel features disclosedherein.

We claim:
 1. A spread spectrum communication system comprising: aplurality of base stations each operable for communication with at leastone user unit; two receiving systems each for receiving independently auser unit signal transmitted from a user unit as a direct sequencespread spectrum signal within which an information signal is modulated;and a diversity combiner coupled to the two receiving system forcombining signals received thereby to reconstruct the user unit signal.2. The system of claim 1, wherein at least one of said base stationscomprises said two receiving systems.
 3. The system of claim 1, whereinone of said two receiving systems is provided in one of said pluralityof base stations and the other of said two receiving systems is providedin another of said plurality of base stations.
 4. A communicationssystem comprising: a first cell site unit configured to communicatethrough a direct-sequence spread-spectrum digital wireless link with atleast one mobile unit; and a second cell site unit configured tocommunicate through a direct-sequence spread-spectrum digital wirelesslink with at least one mobile unit, and wherein the first and secondcell site units are synchronized to each other.
 5. The communicationssystem of claim 4, wherein the first and second cell site units aresynchronized to each other by way of a common time reference.
 6. Thecommunications system of claim 4, wherein the first and second cell siteunits are synchronized to Universal Coordinated Time (UTC).
 7. Thecommunications system of claim 4, wherein the first and second cell siteunits each comprise a GPS receiver for receiving a GPS signal andwherein the first and second cell site units are synchronized to eachother by way of the GPS signal.
 8. The communications system of claim 4,wherein: the first cell site is configured to generate a first spreadingsequence for use in the direct-sequence spread-spectrum digital wirelesslink; and the second cell site is configured to generate a secondspreading sequence for use in the direct-sequence spread-spectrumdigital wireless link, which second spreading sequence is asynchronized, time-shifted version of the first spreading sequence 9.The communications system of claim 8, wherein: the first cell site isconfigured to generate a pilot signal from the first spreading sequence;and the second cell site is configured to generate a pilot signal fromthe second spreading sequence.
 10. The communications system of claim 8,wherein: the first cell site is configured to generate a sync signalfrom the first spreading sequence; and the second cell site isconfigured to generate a sync signal from the second spreading sequence11. The communications system of claim 8, wherein: the first cell siteis configured to generate the first spreading sequence using a firstcode polynomial; and the second cell site is configured to generate thesecond spreading sequence using the first code polynomial.
 12. Thecommunications system of claim 4, wherein the first and second cell siteunits each generate a synchronized PN sequence.
 13. The communicationssystem of claim 12, wherein the synchronized PN sequence generated byeach cell site unit has a chip rate of 1.2288 Mchips/sec.
 14. Thecommunications system of claim 12, wherein the synchronized PN sequencegenerated by each cell site unit has a length of 32,768 chips.